My truck is. 2005 4x4 Silverado lifted with 22x12 with 33s drives smooth until 45mph at 45-50 the rear seams to bounce a lot. If I slow done under 45 it stops if I speed up over 50 it stops any idea what this could be

 

That's odd, but since its lifted, its hard to confirm things. If both axles are at the same ratio, and you have an automatic transmission, it MIGHT have something to do with the transmission. I could see a stick-shift possibly doing something like this. Did you add a locking differential to your 4x4 Chevy pickup?