Vibration - dealer can't find or fix

[dcc2u]

I've got a vibration that seems to start at 62 mph. It's a 2wheel drive, stock, no modifications. The vibration gets progressively worse with speed from there. The dealer (I know, but I'm new to the area) replaced: driveshaft, u-joints, differential (with a used one), and the vibration still there.
I can't feel it at the same engine RPM in neutral. Had wheels balanced, switched them front to rear, vibration seems like its not in the steering, more like in the seat.

Any other ideas? I've read the other vibration threads but haven't come up with something else to check.

Also, Need a recommendation for a decent shop in S. Florida (Miami side).

 

[JWaldz]

Where does it feel like the vibes are coming from? Just for kicks, have you tired wiggling the front wheel to see if the wheel bearing or tie rods are worn? Could also be the steering rack. I know you said it doesn't feel like it's in the steering but eliminating every possible solution regardless of 'feel' is always good to do.

Try this if you haven't already: one at a time, jack each front tire off the ground. Grab 12:00 and 6:00 on the tire and try to wobble it back and forth. There should be absolutely no play. If there is, then it could be a bad wheel bearing. Now grab 6:00 and 9:00 on the tire and wiggle it back and forth. Very minimal play is ok, but excessive play is not. If it's wiggling a lot have someone look at the tie rods and where the tie rod connects to the steering rack while someone else wobbles the tire back and forth. If it wiggles just in the tie rods, then replace the bad tie rod. If it wiggles in the steering rack (removing the boot to see condition of inner tie rod might help) then it could be your steering rack going bad.

At this time, you could also check to see if the bushing on the rack are worn out. With front tires on the ground have someone jerk the steering wheel back and forth and see if the rack itself has any excessive play.

Another thing that causes vibrations is a bad rear pinion bearing. Given the fact you just replaced the rear 3rd (diff) with a used one doesn't necessarily mean it couldn't be bad either. A good way to test the pinion bearing is to get under the truck, grab the rear of the driveshaft where it connects to the diff, and move it up and down/side to side. If it moves around or makes any pop noise, it's toast. Look on youtube for "bad pinion bearing" and you'll see what I mean.

This might all be moot cus it's already been tested but it was worth a shot to give you some insight on some DIY tests.