833 OD vibration

I have a vibration from 2500 to 3000 rpm (engine speed) in 4th gear. I cant figure it out. I have tried everything. I replaced the u-joints, slip yoke, and rebuilt the trans with new bearings and output bushing. swapped in a different rear end. tried changing the u-joint angles. had the drive shaft balanced. put the car on stands. removed the wheels, drums and axles. I know its not the engine because the vibration is dependent on output shaft speed. in 3rd gear the vibration moves to a higher rpm (approx. 3500). I even removed the slip yoke from the drive shaft, then installed it in the back of the trans and ran the engine to 3000rpm. the vibration was still there, only much less noticeable. has anyone ever seen a trans issue that could cause this?