I know this has been on the forums before, but here is a recent version. Like everyone, bad vibration at highway speeds - sounds like angry bees.
Took it to a Ford dealer that has a large truck center, they installed the Ford TSB which is some kind of balancer in one of the drive shafts. It didn't work. They tried everything they could with the drive shafts installed at the chassis shop and gave up. They brought in a drive line specialist and he replaced the 2 non-Ford driveshafts, balanced and phased all three, and set shaft and differential angles. The result is smooth as silk. The photos are before and after. Clearly whoever is doing this work at the chassis shop has no clue. If i did it right, before and after pics are attached.
If your unit is under warranty, don't wait - get it fixed. This was a $1700 repair.