UrQ Grinding Sound - Help

[Andrew Finney <afinney@tamsconsultants.com>]

Well, it's about time to make the first major sacrifice to the
Audi Gods (tm), besides all the thousands I spent on bombs,
brakes, chips, switches, cv joints, bushings, repainting, etc. 

Problem - Happened for the first time in early December. Going
uphill with both diffs locked in the snow, shifted down from 4th,
suddenly loud vibration, whirring sound,  sickening noise.
Slowed down, unlocked diffs, car ran fine.

The problem reappeared last night. Diffs locked on the highway
in the rain, Under low rpm acceleration at 50 mph the car
made a lot more noise and slight vibration, almost sounded
like an exhaust leak. Then suddenly it make a loud thunk,
thunk noise which almost felt like the driveshaft hitting the
tunnel. Unlocked diffs, crawled home. Now the car has a slight
whine when driving around town.

I looked at the center carrier bearing two weeks ago and it
seemed fine, no rips in the rubber. I'll look again today if it
stops pouring. I'm not sure what constitutes excessive play but
it didn't move much except for the rubber compressing.

Other notes. I think the right front motor mount is shot. I have a
lot of vibration at cold idle and shudder when engaging the
clutch into 1st gear.  Synchros in 5th gear are also on their
way out. On long trips I have to sometimes slam the lever to
engage 5th gear.

Is this a tranny problem, a center diff problem, or a carrier
bearing problem? It only seems to manifest itself when the diffs
are locked but I think that's going to change and I'm going to
be left stranded.

It's my only car - RDH.

Thanks for any input, I fear the worst.

Andrew Finney
1983 UrQ.