During acceleration between 40mph and 60mph, the vehicle shakes. When throttle is released, shaking stops. Acceleration above 60mph, no shake. Tires and wheels are balanced. What else could be causing this?

 

Has it been raised or lowered? Sounds like u-joint.

 

Like faawrenchbndr said, maybe u-joints or a drive shaft problem, Chock the vehicle, put in neutral and push up and down on drive shaft to see if there is any movement, also comes to mind is engine mounts or the transmission mount may be loose. But it does sound like a drive train problem. Please remember I said CHOCK the vehicle before getting under in neutral. If it's u-joints you should probably notice vibration under de-acceleration also.

 

I have the same problem.... ive been trying to figure out what it is.... hopefully one of us finds something out and can alert the other

 

Are you runing OR tires?