I had the exact same problem with mine. Vibration between 50-60 mph. Brought to a shop, balanced all 4 wheels. No change. Put it up on the rack and the tech narrowed it down to a bad drive shaft...... 
Well I brought it to a friend and hopped under the truck for 2 seconds grabbed the CV shaft and it was loose. So I ordered 2 new CV shafts, all 4 ball joints, outer tie rod ends, and sway bar links.
Vibration is gone. Drive almost brand new now.
Edit: BTW I'm glad it wasn't a bad drive shaft, I can't imagine that's an easy or cheap job.
Well I brought it to a friend and hopped under the truck for 2 seconds grabbed the CV shaft and it was loose. So I ordered 2 new CV shafts, all 4 ball joints, outer tie rod ends, and sway bar links.
Vibration is gone. Drive almost brand new now.
Edit: BTW I'm glad it wasn't a bad drive shaft, I can't imagine that's an easy or cheap job.