I haven't worked on that many differentials but my understanding is if it was quiet at one time and the noise developed gradually, it is gear noise or noisy bearings, but not a problem with the initial setup and adjustments. Checking the tooth contact pattern with a special ink will tell if the area of contact is correct. If it's the cause of the noise, the howl is normally constant and steady, and will change with load. Bearing noise usually has more of a pulsing howl that gets louder once per wheel revolution.
If the pinion bearings are bad, (pinion gear is the one hooked to the drive shaft), there will often be leaking gear lube right behind the rear universal joint. Sometimes you can move it up and down a little too.
Replacing the gears is pretty involved. It can require the use of a case spreader tool and a shim kit. The contact pattern has to be checked, then everything is disassembled to try a different thickness shim, then it's all reassembled again, ... Many times, until it's right. Because sometimes the cause of the noise is hard to diagnose, some mechanics automatically replace the gears and all of the bearings. That gets to be very labor-intensive and costly so it's much more common to just get a used axle assembly from a salvage yard. Differential noise is not a common problem so your chances of getting a bad one from the salvage yard are small.
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Wednesday, June 20th, 2012 AT 7:26 PM