Pilot bushing in the crank ! I had the same problem the input shaft would wobble in the crank, in 4th gear it's straight through so the tail shaft wobbled causing the drive shaft to wobble in the front and it set off a bad, bad vibration. 3rd gear it's not straight through so the wobble would cancel out. change the bearing and it went away for a while then come back. I found the bell was off center and would wipe out the pilot after a short while. I had to put in offset dowels .020 and that fixed it.
