4000qtr driveline query

[Jon Blake <jonb@u.washington.edu>]

I'd like to post this for the collective group wisdom:

Car: '86 4000CS quattro, 120K miles

Symptom:  under load, at speeds above, say, 50 mph, muffled thudding
coming somewhere in the driveline. Sound goes away with lifted throttle.
I'm thinking u-joint or perhaps the hanging bearing through which the
driveline is routed to rear.

Any thoughts?  Typical maintenance/repair item for car with this mileage.

Obligatory car rave note:  Have had this car since new and have used it
often to traverse Cascade mountains in Washington state. Over this
Thanksgiving day holidays the car handily took care of driving rain,
freezing rain, heavy snowfall, wind driven snow with gusts of 45+ mph.
Easily went up unplowed road to cabin, with 8" new snow. Was worried I
might high center it, but did just fine. First time I really had to lock
the diff. This has been a terrific car for me.  It was even gracious
enough to have its fuel pump go out the week before the trip so I could
fix it at home rather than strand me in the mountains; can't have a car
take better car of its owner than that!

TNX for any and all thoughts on my latest perplexity - jon