Strong vibration at 50+ mph
 

So in my 95 f150 (inline 4.9) there is an intense vibration noise. It's almost unbearable at interstate speeds and I feel like it's worse in 5th gear. I don't feel it in the steering wheel at all and really can't feel any vibration anywhere other than sound. Crazy thing is, when I push in the clutch, the sound goes away. It's perfectly quiet when I'm not accelerating and the engine isn't under any load. It has a brand new clutch kit (pressure plate, slave cylinder, pilot bearing, clutch) which I needed anyway but I thought it would solve this problem. Looked around and some said out of balance dshaft, or bad ujoints but like I said...when the clutch is disengaged it's is perfectly quiet. Thinking maybe an engine mount?

Driveshaft u-joints, most likely.

Crawl under the truck, grab ahold of the rear driveshaft and see if their is any play in the u-joints - there should be absolutely none whatsoever.

Another culprit could be the transmission mount - another thing to inspect while under the truck.