Alright, I have a 1987 F150 with 4.9l I6 and granny gear 4 speed. Today while cruising around town i noticed a harsh rumble coming from my truck. It only happens in 4th gear from 1300-2000 RPMs. The sound mostly resembles that annoying rumble you get when you start ascending a hill in 4th or 5th gear and need to downshift, however this is happening to me when Im just driving. Any ideas on what this could be? Any help would be appreciated.

 

Also truck is 4WD. I checked transmission mounts and motor mounts and they seem fine. Maybe U-joints?