Tires: Rear Vibration @ 60 to 75 mph, chevy silverado, factory rims

Question
I have a 2000 Chevy Silverado (Short Bed). I purchased this truck and replaced the stock 16" wheels and tires with 20". The truck would ride okay at low speeds but when i get up between 60 - 75 mph I feel quite a bit of vibration. Sometimes when I get above 75 mph the vibration lessens a bit. The wheel dealer said the rim was moving in the tire and throwing it out of balance. I marked the tire, nothing moved. I rotated the tires at 4000 miles and the vibration continued to feel like it was coming from the right rear.I replaced the tires recently and still have the vibration in the same area, Right Rear! I don't remember the
factory rims having the vibration, I'm gonna slap them on the back today, and see what I get. Any imput would be greatly appreciated. Thanks Ty

Answer
Ty,

I'm a little unclear, but when you felt the vibration was it always coming from the Right Rear?  If so, then it is either something in the axle or the way the rim mounts on that position.

The other possibility is that the U joints are causing the vibration.  Sometimes it's the pinion angle that is the culprit.

Hope this helps.