Originally Posted by Rzeppa
Definitely sounds like u-joint based on symptom description. Vibrating at a certain speed under a certain load (or opposite - when coasting) has always been the hallmark of every worn out u-joint I have ever had.
I agree 100%. take the DS of and inspect the U joint of the truck. On the truck it can still seem to be good if it is just starting to go. Once it is badly worn it will squeal and vibrate at lower speeds too.