When I hit approx. 80 mph I get a bad vibration in I can feel through my seat not the steering wheel, it goes away at about 92mph or if I put it in neutral while duing that speed it stops. Does anyone have any idea on what this could be??? 87 5.0TPI auto

 

I've heard that the if the stock driveshaft is out of balance that high speed driving can be a effected, but I have no personal experiance with it.

 

"personal experience with it"
Sounds like the drive shaft or universal joints