Hmmm...I wouldn't change the trans just yet.

It's possible that the drive shaft is out of balance. It could have threw one of the weights. I'd get under there and check it out.

Or it could be your wheels are out of balance. Try swapping the front tires to the rear and see if the vibration changes.

If you shift the truck into neutral at 70mph, does the noise go away?

88 4R, 350 V8, R150 5 speed Sold <...uot; />
97 4R, stock, daily driver
98 Sienna, kid and dog hauler, wife's ride