Hi linzwvu. Welcome to the forum. It is not necessary to replace both springs, but it is also not neccesarry to buy two new socks or shoes either. One new shoe is sufficient if only one old one has a hole in it.
I know that sounds goofy, but springs are the same. A person would replace just one spring if the truck was very new or very old. They will all sag a little over time. They haven't had time to sag yet on newer trucks so replacing just one spring can be justified. On older trucks, people are concerned with saving money and performing repairs as cheaply as possible so they might replace just one spring, or one tire, etc.
Ride height is critical for proper handling, braking, crash avoidance, and tire wear. Even when tires are in perfect alignment when standing still on the hoist, sagged ride height makes the suspension parts go through incorrect arcs as they go up and down over bumps in the road. This is more important for cars, and really doesn't apply to solid rear axles such as on your truck. It also doesn't apply to older trucks from the 1990s with solid front axles, but it does apply to newer trucks with independent suspension. That is a weaker system that is used to give trucks a smoother ride that people demand.
To get back to the issue, replacing both springs insures the truck will sit level and bounce over bumps evenly. One strong spring and one weak spring will make the truck feel like it is dancing around on rough roads.
It should take about an hour per spring to replace them, possibly a little longer if bolts are rusty. Most reputable mechanics will also replace the four u-bolts that hold the axle to the springs. As a former suspension and alignment specialist at a very nice Chrysler dealership, I was not involved with parts prices, but I can offer a suggestion that a pair of springs, four u-bolts, and labor could run around $300.00 to $600.00. An estimate for $1000.00, in my opinion, would be cause to seek out a second estimate from an alignment specialty shop. Every large city has a few of them.
An alignment is not required, however most mechanics will do one to insure the steering wheel is straight. It is typical for the new leaf springs to reposition the axle slightly. If one wheel is moved forward or rearward a little from where it was, that will throw the front wheels off a little to keep the truck going straight. That is common. The neat thing about Dodge trucks, at least up through 1999 when I left the dealership, is they have one adjustment that only affects steering wheel position. I often set that adjustment by stopping on the side of the road during a test drive. That can be done by itself or as part of an alignment, and can take a few minutes or much longer, depending on how picky the mechanic is. Around 2001 and 2002, Dodge began switching to a different steering system on the lighter trucks that required an alignment to position the steering wheel straight ahead.
Thursday, June 17th, 2010 AT 8:16 PM