Hi KEn,
did you check the center bearing?

this bearing is located in the center of the shafts on all type44 quattros and is surrounded by a rubber insulator.

if the rubber cracks, breaks, wears out ....etc..
the bearing starts ridding on the bearing support and vibration is transferred over to the Chassis ...BUT noramlly for you have this you have to do allot of Miles/Kilometeres to see this kind of event.

You can buy the center bearing separately from one company in the US ..BLAUFERGNUGEN!!!

If you need more details let us know.

Paul