Evil Quattro Driveline Noises...HELP!

[Audial <Audial@aol.com>]

Well, they say never to praise anything out loud...

After my wonderful trip back and forth to the East Coast, my '86 5KCSTQ is
sending shivers up my spine today.  It is making very nasty sounds with the
center differential locked.

During my trip on mostly straight highways, I had locked the center diff on a
couple of occasions when I hit snow.  I had always disabled it before taking
any off-ramps or sharp corners, so the problem didn't manifest itself.

Today there was some heavy rain in Chicago and so I locked the differential
again for some parts of my trip.  On one occasion, I didn't turn it off when I
left the highway.  At the first moderately tight turn, I began to hear and
feel a grinding and a vibration emanating from the area aft of the shifter and
the floor between and rearward of the front seets.  It starts as a vibration
and a _loud_ metallic grinding noise that sounds almost as if the driveshaft
is rubbing on the exhaust system.  Turning the wheel a few degrees more causes
the grinding to transform into a pulsing/throbbing vibration that sounds like
it's occuring each time the driveshaft turns.  It is _loud_ and sounds like
someone is rapping on the floorpan with a hammer.  It occurs in every gear.

With the diff unlocked, the driveline is completely normal - no strange noises
in any gears including reverse, smooth turning at full lock, no shuddering,
etc.

On less acute tuns, I never hear the noise.  I realize that there will be some
vibration normally with the center diff locked while making tight turns, but
this is another thing entirely.  

Does anyone have any idea what might be causing this?  The engine and
transaxle mounts were OK the last time I checked and the car drives very well
under normal circumstances with the center diff unlocked.  

I fear the worst.

Thanks for any news good or bad,
Best WIshes,
Alex