Hey man, I figured out what it was on mine. It was the u-joints on the front shaft. Put it in 2 high and see if it still does it. When I would have it in auto 4wd, it was a medium vibration. When I would have it in 4 high, it was TERRIBLE. And there was no vibration in 2 high. Try that.

 

he took the front driveshaft off , so thats not it. flexplate (flywheel) could be warped or possibly bad converter. no other ideas. hope u figure it out.

 

I also had the same problem a few months back. My truck would vibrate over 3k rpms while driving. It was the torque converter. However, it would also vibrate sitting still while revving the motor too.
After checking the obvious things I removed the tranny bolts and slid the trans back, then removed the converter (TH350) and started the truck. No vibration.
Just make sure you rule out all the easy stuff first before pulling trans.
Good luck!