I'm just saying what happened to me and what fixed it. In 1st 2nd and 3rd the input shaft is spinning at a different speed than the output shaft, so if the input wobbles the output shaft is turning slower then the vib seems to cancel it self out and you might not feel it but in 4th gear the two shafts are turning the same speed and it is like a straight shaft that gets bent in the middle, now if you put a bearing on both ends and spin it the middle swings out of round and both ends wobble. if it's not out much you might not feel it till it gets going fast enough to set up a oscillation then its hard to stop.
If you put everything in new and didn't change or check the pilot it could just be worn out. Is it a bushing or a ball bearing ?
