i have a 1995 f250 5 speed and here lately when i get to 55mph and just give it enough gas to stay at 55 it vibrates really bad seems like its coming from right underneath my seat i can give it more gas or let off the gas and it will stop it just started doin this couple days ago all the ujoints look good i changed the tranny fluid idk what else it could be your all help would be highly appreciated

 

1994.5 - 1997 7.3L Power Stroke Diesel - Ford Truck Enthusiasts Forums Have you tried asking on this page?
Could be an engine problem and those guys could help.