Since it only vibrates at 50-60 mph, it is most likely the tires, either the capped aspect of the tires, or their balance. As previously posted, swapping front to rear is the best way to determine that, as long as the front tires are not cupped, and you currently have no vibrations up front.

If it were driveline vibrations, you'd expect to feel the vibration starting as low as 10-20 mph and gradually getting more violent as speed progressed from there.

Also, with a 2" spring lift and 2" shackles, it's very unlikely that the driveshaft angle has increased with 31" tires enough to cause driveline vibration.