vibration dilemma

When I accelerate up to about 3500 rpms it is smooth as glass, but after 3500 it starts to vibrate. At 4000 and up it is shaking the whole car and I could only take it to 4500 cause the vibration is too much. It vibrates on decel down to 3500 and then it is smooth again.
The driveshaft is 52.75 inches center to center, 3 inch steel tube, 1350 crosses. I have checked pinion angle and it is good.
Is the diameter of the tube too small and whipping at the high rpms?
I have no clue what to check.
This is on a 72 dart. 650 hp smallblock and a 904 and a 8 3/4 with 2.94 gears.