Vibration ONLY at 75 mph

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.