Suburban 2500 Questions

I will add to the U-joint band wagon. I had a Ranger that gradually acquired a vibration that started about 55mph. Everything looked fine. When I removed the driveshaft, the u-joint connecting to the axle, one direction of movement was ok, the other had a bind. U-joints are cheap and relatively easy to replace. If the shop does not explicitly state they replaced it, I would.

Also, could be a loose inner or outer tie rod. If there is any play at all, it will quickly cause a uneven wear spot on the tire, which then will cause a vibration. Been there done that with my small car.