Vibration in drive-train?

[Brent Henry]

My neighbour has a 1995 A6Q... he has noticed over a month ago that one of the outer CV boot's has been torn.  A week ago he noticed that at full turn, it is now clicking.  A few days later, he noticed a vibration in the drive-train -- transmitted into the driver seat -- not the steering wheel.  The next day, the vibration got alot worse, it is severely exaggerated when accelerating (even slightly).

He is now in the process of changing the CV joint and boot-kit, but he is concerned that the vibration is not coming from the CV joint?  Considering that it came on so quickly and got so much worse in just one day?  I know that Quattro drive-train's are rock solid, but is there any chance that it is something to do with the drive-shaft, u-joint, rear diff, etc, etc???  

Thanks,
Brent Henry.