I just picked up a 95 F150 reg cab SWB & its got a weird vibration that I'm having an issue trying to track down, here's what it's doing & what I've done trying to trace it down:
Starts around 15mph, continues pretty hard til around 30, then gets better, only under acceleration & is still there at higher speeds, but not as noticable.
I bought the truck used, its got 77k miles, is a 5.0, auto, it already had new front brake pads & rotors, rear shoes, drums & u-joints
I have replaced the front tires (1 was sepo'd), replaced the rear drums & fixed the rear brakes (r/r shoes were reversed & had a warped the drum)-no help under acceleration, helped a braking issue
sent the driveshaft out & had it rebalanced-no change
swaped rear tires out for other set-no change
checked motor mounts, all seem to be good
check rear end, gears, bearings all appear ok, no chipped teeth, no noises
it's got me stumped....HELP!

 

Do you feel it in the steering wheel or the seat/floor board?

 

Through the seat, and increases with the speed of the truck & not by engine RPM

 

Sounds like the feeling you get with a separated belt in a tire or driveshaft issue. I know you had them checked/swapped but first things came to mind. Is it completely gone when not accelerating? Bent diff yoke, trans output shaft bushing, tire balance?

 

I thought it was a driveshaft too, the truck had new u-joints when I got it, I had the driveshaft rebalanced, no change, I've swapped tires around & rebalanced them, I've not checked the trans yoke, but I dunno how it could've gotten bent, thats next on my list
Thanks for the help, I'll keep the post updated