my truck vibrates really bad at 20 mph or less, just started this. it completely clears up over 20. i was thinking it has something to do with the rear end. noticed there has been some fluid leaking out of the area where the driveshaft connects to the diff. so checked the fluid level and its low. i'm going to change it today and get another seal for the diff also where it connects to the driveshaft. any other thoughts would be greatly appreciated.

 

Yeah...your u-joints...check them carefully for brownish powder near each bearing seal.
My truck was vibrating like that at low speeds also....turned out that one of the u-joints could not even budge(turn) by hand. and it wasn't very obvious that the joint was bad at first by just looking at it.