Drive shaft vibration should be a factor of road speed not engine RPM. Most of my older Quattro's (2- 85 4K's and one 88 80Q) had a vibration around 35~40 MPH. My 98 A4 has no vibration even with 200K mi on it. Make sure the vibration is due to the drive shaft (road speed) not engine speed. You could have a different problem.