Got a weird issue for you guys. 2002 v6 just completed a manual trans swap. Now I've got a nasty vibration that starts at 65 mph... But only in 5th gear. When I'm in 4th gear the vibration is very minimal. When I shift to 5th and get up to to 75 it makes a loud noise and a nasty vibration. The faster I go the worse the vibration and louder it gets. Noise seems to be coming from the engine almost sounds like a bad misfire but the rpms aren't fluctuating. No check engine light. I can count the rumbles (80 rumbles every 50 seconds or 1 mile traveled @75mph). By the time I get to 80 mph the rumble is constant. I suspect a bad 5th gear or u-joint. (Junkyard t5 and drive shaft). Any thoughts?