GM-GMC: Rear End Grinding Noise, gmc vandura 2500, rear axle assembly

Question
It's not a diesel question but its definately truck.

I've got a well-used '93 GMC Vandura 2500 with 127K miles (10K of which are mine).   I don't know much about rear ends - I know it's a limited slip but don't know offhand what kind - strictly stock. I started hearing a "wugga-wugga" noise coming from the rear end about five months ago.  I've had the drum brakes totally replaced, wheel bearings replaced, rear differential fluid changed, tires replaced (they did need to go).  I asked about the axle bearings and two mechanics glazed over on me with the "whut?" look.  Last night I finally put the van up on jack-stands and crawled under with a stethescope.

The noise is directly proportional to wheel rotation, but is not isolated specifically to one side, at least by listening.  I stopped both wheels (one at a time) and the sound seems to remain proportional to wheel speed, no matter what wheel was turning, but is definately strongest in the center section of the differential casing.  It is more dominant when driving on right turns than on left turns, but happens at all speeds.

I've already sunk about $750 into getting things fixed that didn't fix this trouble (when the goal was to fix the noise).  Suggestions I've heard include getting a junk axle - but I'm reluctant to have something as critical as the rear axle replaced with a junk part - plus I'm no longer able to devote time to ripping apart and refurbing a beat-up van.

I can't find an exploded diagram of the rear end to even understand what the trouble could be.  The next step is to take it to a GMC repair facility (since the local mechanics can't find the trouble or aren't tooled to fix it).

Please help.

Thanks
Dwain

Answer
Unless you want to spend at least what you have spent already, I suggest you reconsider a used rear axle assembly from a salvage yard.

Messing with the center section of the differential requires special tools and years of experience to set everything up correctly.

A good used axle (many salvage yards will give a warranty on their parts) can be installed in much less time and will cost much less money.