Drive line vibration

I have a driveline vibration that only happens when I accelerate slightly when travelling at highway speeds. After I ease up on the gas, it goes away. Only present when the driveline is under load at 55 to 7o mph. I replaced the front prop drive shaft-30 inches or so long. Stiil have the problem. Any ideas?
Thanks.
Bob

Vibration is very difficult to diagnose without driving the vehicle first hand, even for an experienced technician. With that said... what you are describing could be a worn or loose universal joint in the rear driveshaft. Or possibly the driveshaft is out of balance.