Because the vibration only happens above 70 MPH it has to be coming from some where between the output shaft of the transmission and the wheels. I have been battling a similar vibration for years. With the drive shaft out try moving the yoke through it's full range of motion. If it binds even a little it will cause the shaft to vibrate at higher speeds. This will not show up when they balance the drive shaft. After changing U joints mine still was binding. It turns out the two flanges of the yoke that the U joint mounts in were not parallel, it was Y'd out. A new yoke solved the problem. It is an easy check.
