well, if i may add my .02 here. (take this with a grain of salt, as i am just guesstimating). was there any metal in the fluid when you changed it out in the rear diff? Also, I have never bothered to look under my car, but does your car have a 2-piece driveshaft? If it does, maybe check the center bearing on it. Otherwise, bring it to a different Audi shop and have them double check the rear bearings.

 

Can you feel any play in the hub? I like to check with the wheel on the ground and with it in the air. With the car on the ground, grab the tire at 10 and 2 and shake the crap out of it. There should be zero play. Bad bearings will have a light "click" that you can hear/feel while shaking.

Then, jack up the car and repeat, although less vigorously so you don't knock it off of the jack and or stands.

 

The fluid was clean, and yes its a 2pc. driveshaft and the center bearing was good the last time I checked a few months ago.

I already checked while on ground, but the car is going on jacks tonight for further inspection.

 

Does the sound change if you on power or coasting? How about engine braking? Can you only hear it at high speed or all of the time? Does it seem to change with engine speed or wheel speed?

How about when the brakes are applied? I thought that I had a bad bearing in my RX-7 once that turned out to be one of the rear calipers dragging.

 

This is all projected to wheel speed...

Placed it on a lift last evening to find out it is indeed the right rear bearing.
Also checked the control arms and tie rods for play and they're all fine.... so this would mean either the shop did an incorrect install or its a faulty bearing.
Now the fun begins and I'm thinking of whether to replace both or just one...