Ok so I've had a bit of trouble today. The vibration starts around 45 and from there it gradually gets worse as your speed increases. Before 45 it feels normal but you start feeling a small vibration at 45 and by 55-60 it almost feels like you are in a massage chair. As soon as you let off of the gas it stops completely. I thought it could possibly be the transmission but at lower speeds it's fine, its never slipped, and I was driving along at 55 MPH and shifted from D to 3 and the vibration felt exactly the same. The strange thing is that I drove the truck two times, later yesterday afternoon and a couple of hours ago, and it seems to have stopped doing it all together. I was thinking u-joints but it's strange that it would have stopped. It has rained and I did drive through some puddles and that would have wet everything down but I'm really confused. There is no such thing as a fluke when it comes to cars and there is always a cause so suggestions are welcome. By the way, what's a good way to test the u-joints?