Growl like noise at about 30 mph?

With stethoscopes, you're going to have to run alongside the truck. You said the noise starts at around 30 mph. I've often imagined strapping myself under a vehicle while a friend drives it, but either my butt will drag on the ground or my nose might bounce up and hit the spinning driveshaft.

The stethoscope in your first photo is what I use to listen next to front wheel bearings when a front-wheel-drive car is on the hoist. You'll think one side might be the culprit, then the noisy side will blow out your eardrums, so it will be obvious.

The Chassis Ear on the right is the model I found a few days go. It appears to use six wired microphones, like mine, but the amplifier is very different. Mine is much smaller and must be used with head phones. It looks like this one might also have a speaker in it. That would be nice. You can't beat the price. The "Buy it Now" is a quarter of what I paid for mine years ago.

The only thing you can see visually is chewed up bearing races, broken teeth, and large metal chips. If there's a speed sensor on top of the differential housing, it is not uncommon to find very fine metal particles stuck to its magnet. You won't see a noisy bearing.

As for the replacement, check with the people at any salvage yard. If you've never used a Hollander Interchange Guide, they likely have them on computer now, but they used to be really large books that listed every part on every car model, with a code number. You look that number up in the back of the book, and there it will list every car model and year that could have used that part. That will cover any physical dimensions and mounts, parking brake cables, and things like that. If you get a complete axle assembly, you also have to look at the size of the brakes. Very often there's two or more that were used. If an axle is available with different size brakes, ask if the brake assemblies can be transferred from your vehicle to the replacement axle. Often you can get an axle to work that has larger brakes, but that is a very bad idea. The front and rear brakes are carefully sized to form a balanced system. Switching to larger rear brakes will make them do too much of the stopping, leading to easy rear-wheel lockup and loss of steering control. Switching to smaller rear brakes will force the front brakes to do most of the stopping, leading to front wheel lock-up with reduced stopping power. With skidding rear tires, the back end will want to come around and pass the front end. With skidding front tires, you lose steering control. Also, if you're in a crash when the other guy ran a red light, a lawyer or insurance investigator will correctly convince a jury that you were partly at fault because you were less able to avoid the crash. Once they find a modified brake system, they'll work extra hard to pick your truck apart and find anything else they can to shift blame away from their client.

You also want to find an axle with the same gear ratio. That ensures the speedometer won't have to be reprogrammed, and you'll have the same shift characteristics you're used to. The ratio is stamped on a tag that's under one of the rear cover bolts. It's also stamped on the outer surface of the ring gear. You can find that by rotating it once the cover is removed. Trucks often had a list of options inside the glove box. I don't know if they still do that, but that is where they used to list the gear ratio.

As for mileage, solid axles have such an extremely low failure rate that it wouldn't be a concern to me. I have a friend with a body shop who specializes in rebuilding smashed one and two-year-old Ram trucks. He regularly drags two home at a time on a trailer from 800 miles away. The trucks he builds for himself typically get sold when they reach 400,000 miles. In over a dozen trucks with that mileage, the only one he ever replaced the rear axle on was one he turned into a dual rear wheel truck.

I forgot to mention earlier to keep in mind if you're trying to find a noise coming from the axle, if you're doing that with it up on jack stands, with a stethoscope, nothing is "loaded", so most noises won't be generated at that time. There's no weight on the axle bearings, and no torque on the pinion and carrier bearings. When you're driving down the road, the noise will change in intensity between accelerating and coasting. Axle bearing noise will not change.

Water contamination is likely from driving through deep water. Water gets in through the vent on top of one of the axle tubes.

If you don't have a tag under one of the cover bolt heads, the gear ratio is stamped on the ring gear. I put a red box on your photo showing its location. Those numbers are usually very small, as in 1/4" high.

I can't be of help there. My experience with axles was back in the 1970s with cars I was racing. At that time, Chrysler had five axles. The 7 1/4" was tiny and only used in the Darts and Demons. I had a '72 Dart that developed gear noise. I replaced it with an 8 3/4". And went from a 2.72 ratio to a more common 3.23. Engine speed was a little higher, but fuel mileage on the highway stayed the same because there was less load on the engine. Made for blazing acceleration on the race track. These were 318 c.I. Cars. Your 3.9L is a 318 c.I. With two cylinders lopped off, so the 7 1/4" axle will work fine.

The 8 1/4" axle was the second smallest I never paid attention to the number of bolts in the cover, but that can be a means of identifying which one you have.

The 8 3/4" was the only axle of the five that you dropped the drive shaft, pulled the axle shafts, then the differential dropped out the front and you could work on it on the work bench. No rear cover on that one. This was the most popular axle. I currently have four in '72 and '73 Challengers. They hold up fine to 340s.
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The 9 1/4" was next, commonly used on police cars. I had one of those on a '77 Monaco 440 c.I police car. They used them too on the Little Red Express trucks.

There's a lot of variables there you don't have to worry about unless you have some donor vehicles in the back woods. If you get an axle from a salvage yard, they will know very quickly what you have and what they have to replace it. If you want to switch to a larger axle, you need to consider the dimensions of the attaching points, spring mounts, brake size, pinion gear depth, and parking brake cables. If there was an optional axle used in your vehicle, and you find one in a salvage yard, it should drop right in, but remember the drive shaft length. If the pinion gear is longer, you'll need a shorter drive shaft. If the axle comes from a four-wheel-drive, the donor's driveshaft will be a lot shorter. Also look up the part numbers for the universal joints. If they are the same for your truck and the donor truck, the yoke will be the same and you can reuse your u-joint / drive shaft.

In the swap from my 7 1/4" to the 8 3/4", the donor axle was also from a Dart, so everything matched. The brake backing plate design was different, but the brake lines, parking brake cables, rear brake flex hose, and leaf spring mounts were all exactly the same. No modifications needed. The brake drum diameter, which was critical, was also the same size, even though the shoes were different.

My suspicion is the numbers on the left photo are internal part numbers used by the axle manufacturer and have no relevance to this story. On the right photo, numbers are commonly painted on during assembly to denote shim or spacer thickness, gear backlash in thousandths of an inch, tooth clearance, and things like that.

The people at the dealer's parts department are the best at decoding the door stickers.

You're over-thinking this if all you want to do is get a different axle assembly. Tell the people at any salvage yard what you want, and they'll know which relevant questions to ask of you. Mainly that is going to be gear ratio.

As for tire sizes, almost all vehicles are available when new with optional sizes right from the factory. In the case of '88 Grand Caravans, for example, 99 percent of them listed 14" on the door stickers, but about 10 percent actually came with 15" wheels which included larger brakes. I have one of those as did one of my high school teachers. My doctor also had an '88, but with the 14" wheels. I have an '89 too that lists 14" wheels, and that's what's on it. Since most that came with 15" wheels still listed 14" on the sticker, I have to guess they ran out of the correct stickers, or they only printed them for the standard tire size. On your truck, the dealer can tell you if they came with optional 16" wheels. If not, then yours were installed by a previous owner.

On a related note, there is some confusion when it comes to tire pressures. Use the numbers on the door sticker as a guide, but that's only a starting point. Ford got into a huge legal nightmare a few years ago by recommending pressure that were too low on their Explorers, leading to a lot of blowouts. The numbers on the sticker are selected for a compromise between tire life and comfort. Where the problem comes in is they have no way of knowing what you're going to buy for replacement tires. In the case of Chrysler's cars in the 1990s, the first thing I did during an alignment was to set the tire pressures. I never looked at the door stickers. Instead, I went by the tiny numbers molded into the sidewall of the tires. Some tire brands specified 35 psi. I set those tires to 35 psi. Other tires specified a maximum of 44 psi. I set those to 40 psi. In both cases, the cars handled well and I never had a complaint of ride quality.

Where the confusion comes in is if the car came with 44 psi tires and they printed 40 psi on the stickers, they would have a serious liability issue if you purchased 35 psi tires later. Now, if you go by the sticker, your new tires would be 5 psi over-inflated. Besides accelerated wear in the center of the tread, the sidewalls would be less tolerant of an impact. The pressures listed on the stickers have to take into account any brand and model of tire you could purchase later.

Those pressures are also selected based on the weight of the vehicle they will be holding up. Switching a tire from one car model to another often requires a change in tire pressures, especially when putting them on a heavier car. Tires will get warm when driving, and that makes their pressures go up. The maximum pressures listed on the sidewall are for "cold" tires. "Cold" does not mean winter. It means when the car has not been driven for many hours. If you check those pressures right after driving a considerable distance, they can be expected to be a little high. That's okay. I had to take that into consideration during alignments and oil changes. If the car was dropped off yesterday, I set the tires to 35 or 40 psi. If the customer just showed up a few minutes ago, I set the tires a couple of pounds higher.

I wouldn't disassemble anything. It was working in the donor truck, so it should be fine now. If it turns out to have a problem, they may not allow you to return or exchange it if you even had the cover off.

Axle bearings and seals cause such little trouble, you're more likely to cause a new problem by replacing unneeded parts. Just pop the assembly in and buzz off into the sunset.

If you don't know what the gear ratio is on the old axle, be careful on the first test-drive because the speedometer might be off. On older models from the 1980s on back, there was a gear on the end of the speedometer cable where it went into the transmission. That gear had its number of teeth molded onto it. You selected the gear by the number of teeth and / or by its color to match the axle gear ratio and tire size to the speedometer.

On newer models starting for sure by 1989, and on some a little later, when you alter tire size or axle ratio, you have to use a scanner to program the "pinion factor" into the Transmission Computer. It takes a lot longer to describe how to do it than it actually takes to do it. Your truck, being a 2001 model, most likely uses programming to adjust the speedometer, but it's in a year range where my memory is a little fuzzy. Look on the rear, driver's side of the transmission housing, near the tail shaft or extension housing to see if there's a speedometer cable attached there. If you don't see that, you'll need a scanner or a mechanic with one to change the speedometer. Under "Transmission Computer", select "Special functions" from the drop-down menu, then select "Set pinion factor", or something similar to that. That will give you two selections. The first is "Select tire size". Click on your size from the rather long list. Next, click on "Select axle ratio". That will usually list over a dozen ratios even though only one or two might have been available from the manufacturer when the truck was built. That list includes anything you could do for modifications with aftermarket parts.

When you're done, pace the truck with another vehicle and compare speedometer readings to be sure yours is right. I like to do this myself with my own scanners because there have been a couple of times when the actual tire size wasn't on the list, and I had to select something I thought was close. If the speedometer reading was off, I had to start over by selecting a different tire size, then going on another test-drive. You don't want to be bothering a mechanic over and over again with that as it can become too time-consuming. There will always be a tire size on the list that will give you a correct speedometer, even when that size isn't what's on your truck.

Of course, if your old axle is also a 3.55, no adjustment is necessary.

If you want to change the gear lube, balance the housing on a five-gallon pail with the differential inside the pail. Remove the rubber fill plug if you have one on the cover, then rotate the assembly so that hole is down. The lube will drain out into the pail. Rotate it back, then this is a good time to refill it, but don't put too much in for now. Leave it low, put the rubber cap back in, then install the assembly. After you're done, the truck must be sitting level on the tires, to check the fluid level. Pull the plug again, then poke your pinky finger into the hole to see if you reach fluid. Add more gear lube until the level is within a half inch from the bottom of the hole. If the fluid level is too high, it will run down the axle tubes and overload the axle seals. Fluid can seep out even though there's nothing wrong with those seals. That over-filled gear lube will eventually drip onto the brake drums and mix with brake dust. That will make those shoes grab much too hard, leading to easy rear-wheel lock-up and tire skidding long before you've reached much stopping power with the front brakes.

Once gear lube, or any other petroleum contaminant on brake friction surfaces goes through a normal heating cycle, that lube soaks into the porous linings and the porous cast iron brake drums, and will never come out. The only fix for that is to replace all the parts at the same time. Easier to just not over-fill the differential.

Some older axles don't have that rubber plug on the rear cover. Instead, look for a metal plug on the driver's side, right above the webbing. A few require a 3/4" open-end wrench, but by far, most of them need a 3/8" square drive to remove them. In a pinch, you can pound in a 3/8" ratchet extension to do the job, but the much better choice is to use a standard 3/8" square drive tool. They are available in a whole set of standard sizes and are made just for this purpose. The difference is there's no retaining ball that holds sockets on. When you use a regular ratchet extension, that retaining ball often gets deformed or full of impacted rust / dirt.

You won't be able to pour the old lube out that side hole. For this style, you will have to remove the rear cover. I would wait with that until you're sure this axle doesn't have a problem requiring you to return it.

I've never used a phone for this. When you connect a scanner or code reader to the data port under the steering column, it just becomes one more computer on the data buss. They all share data back and forth. You have to select the computer you want to display data for. As for reading road speed, that will be an item on the list of data for the Transmission Computer, Anti-Lock Brake Computer, Air Bag Computer, Body Computer, and probably a few others that need to know road speed. All of the computers have access to that data, but it may not show up on each one's list of data if it isn't something they need to know.

As for the pinion factor, that is set and memorized in the Transmission Computer, so you need a scanner capable of accessing that computer. Most simple code readers can only access Engine Computers. A few of the better ones can access Transmission and ABS Computers, but they also have to be "bi-directional", meaning besides displaying data, they must be able to talk back to allow you to make changes to settings.

The chart below shows the information for gear lube. If you have a locking differential, you should add a bottle of additive, (friction modifier). The dealer's parts department has that in small bottles. It contains "whale oil". It's purpose is to prevent chattering of the clutch plates when turning sharply at low speeds.

Here's the mechanical specs for the 8 1/4" rear axle. That's the most common one. As you can see, there's three gear ratios listed. If you have the less common 9 1/4" axle, I can post the specs for that one too.

For an axle in the vehicle, fastest is to raise both rear tires off the ground and support the vehicle on jack stands. Leave the transmission in "park" so the driveshaft can't turn. Now, grab one rear tire and rotate it by hand. If you can, it is not a locking differential. When rotating one tire forward, the other one will rotate rearward, (the other way).

When you have a locking differential, both wheels will insist on rotating the same way. To rotate them together, you WILL have to take the transmission out of "park" and allow the driveshaft to rotate. When doing that, there is an over-running clutch in the transmission that you will be forcing to rotate too. Those only like to rotate one way, so don't be surprised if you can rotate the pair of wheels rather easily one way, but it takes more effort to turn them the other way. As I recall, the wheels will rotate easily in the forward direction. You would see the same thing if you were to rotate the drive shaft by hand. One way it will turn somewhat easily with just one hand. The other way, you're turning additional rotating parts in the back of the transmission, so you'll definitely need both hands and it will be noticeably harder to turn.

With these designs, there is a set of spring-loaded clutch plates inside the differential assembly that tries to lock the two small axle gears together. That's what makes both rear wheels rotate under power when either one of the tires slips on snow or mud. Think of this as a rather wimpy hand shake. It gets the job done, but the parts can slip if enough force is applied. In the case of the axle, the two wheels / axle shafts need to rotate at different speeds when you go around a corner. Slippage in the clutch plates allows that to happen, while the pressure ensures both tires get some power when one is slipping. Since there's some play in the springs, during turning, the clutch plates like to grab too aggressively until enough force builds up to tear them apart, then they rotate a little, then grab too hard again. That's what makes the assembly chatter on tight turns. The whale oil additive quiets that chattering.

Now that I've shared this wondrous information, allow me to add a few comments of value, for the benefit of others researching this topic, to ward off potential confusion. There are two other locking differential designs that do basically the same thing, but without those clutch packs. One was an old design we called the "Detroit locker", and I'm pretty sure was only used by GM in the 1970s. That one acted exactly like a standard, or common "open" axle; "open" simply meaning non-locking. Once one spinning wheel was rotating a certain amount faster than the other one, centrifugal force would throw out a weighted lever that activated a mechanical catch that suddenly locked both axles together. That was a result strictly of the difference in rotational speed of the two wheels. I don't know what it took to make it unlock, but it had to do that, otherwise any car, including race cars, are very miserable to drive on dry pavement with the two sides locked together.

Regardless of how or when the unit unlocked, there was also a second weighted lever, also run by centrifugal force, that forced the assembly to unlock at and above 35 mph. At that speed you don't need a locking axle, and to be locked solid like this one did makes for a somewhat dangerous, or unpredictable vehicle to drive at higher speeds. So, this design only kicked in when needed, with no drag at other times, and it never interfered with stability or handling at highway speed.

I know even less about this second design, other than what I've learned from other mechanics. This is a differential used on Dodge trucks that has what looks like an engine oil pump between the ends of the two axle shafts. One shaft holds the body of the pump. The other axle shaft drives the pump's rotor. During normal operation, both parts rotate at the same speed, so no pumping action takes place. It's when there's a significant difference in speed due to one wheel spinning, that the assembly pumps gear lube under pressure. That oil puts pressure on a set of clutch plates that tries to lock the two axle shafts together. It takes a little time for that pressurized fluid to bleed off. The slipping should be done by that time, then the assembly unlocks to improve fuel mileage. I think that design showed up around 2005, give or take a couple of years.

Locking differentials are going by the wayside now. Just about every car and truck model today is available with optional anti-lock brakes, which are an add-on system to the standard brake system. When you have anti-lock brakes, you next have the option to add traction control, and if you have that, you have another option to add stability control. Each new advancement requires the vehicle to have the previous option first. They can't be a stand-alone option. The more advanced 4-wheel anti-lock brake systems control a skidding tire by reducing brake fluid pressure to that one brake, but then it reapplies that fluid pressure once the wheel gets back up to speed and the skidding stops. That, "block, bleed, reapply" sequence can occur up to 30 times per second, but 15 is more common. (That's the buzzing you hear, and feel in the brake pedal, during a skid). Since the ABS system can reapply braking power to any one wheel, the add-on traction control system can do the same thing. Rather than relying on a mechanical locking differential, traction control lightly applies the brake to the spinning wheel, causing the tire with more traction to get more power. My 2014 Ram has traction control, and it's very effective. The first car I drove with it was the first Intrepids at the dealership I worked at, and it too was a lot of fun. Tried, but couldn't get it stuck in snow, going uphill.

By the way, to the best of my knowledge, only one truck, an International from the 1960s, was ever available with a locking front axle. That was back in the days when few people were driving 65 mph. If you can find anything with a locking front axle today, I'm pretty sure it is designed for use on dirt where the tires are free to slip. Driving on pavement with a locking front axle would be extremely miserable and tiring.

There's nothing to gain by going to a synthetic gear lube. Any product specified for this purpose will have the necessary additives, except for the friction modifier. Add that only when necessary. The gear lube will get hot during long highway runs. That causes it to thin, and it gets much stiffer in cold weather. One of the selling points of synthetic oils is they're less susceptible to changes in viscosity with changes in temperature, but if there was a real benefit in those products, the manufacturer would have used them on the assembly line, and they would specify that in the specifications. Standard gear lube meets all the requirements for the bearings to get lubricated, the rubber seals to stay pliable, and it stands up to the pressure between the gear teeth. There's also the consideration that the whole idea of a ring and pinion gear set is the gear teeth are sliding across each other. The gear lube has to stick a film of oil that doesn't squish out, to prevent early tooth wear. The synthetic oils will do the same things, but there's usually different additives in them. To be safe, I would only use synthetic if the entire axle has been drained of the old oil first. Don't use synthetic to top off an axle already filled with petroleum-based gear lube. One product might have a detergent, (more so in engine oil), that attacks the seal conditioner in a different product, for example. Mixing some products can destroy the qualities of both of them. A perfect example is mixing GM's Dex-Cool antifreeze with anything else. It turns it into "Dex-Mud" that can clog radiators and heater cores.

I hope I've covered everything. Let me know if I missed anything.