I had a vibration at 55ish until I recently replaced my rearmost u-joint. Vibrations at a particular speed are caused by resonance in the system. At a particular frequency (in case of a drive shaft, RPM) the vibration is greatest at it's resonant frequency, and this can be felt in the cab if the vibration is great enough.
To check your rear u-joint, all you have to do is go and undo the 4 bolts that attach your drive-shaft to your rear diff. DON'T SLIDE THE REAR PORTION OUT OF THE SLIP YOKE UNLESS YOU MARK BOTH SIDES FIRST! Just swivel the rear u-joint and if there's a tight spot, I bet that's your problem. If the rear u-joint feels fine, you could remove the whole rear drive-shaft and check the other joints.
Replacing a u-joint is one of those easy and rewarding fixes. I thought it was fun, and had never did one before.