2006 Ford F150 Rear of truck shakes when reaching speeds of

I recently had 4 tires put on my truck. Now when I reach speeds around 50 or 55, the bed of the truck starts to shake, I can even feel it in the cab of the truck. I took the truck back...they pulled all tires off and re-balanced...even moved them around. It still does it??? It didn't do this before I bought the tires.
Any suggestions???

Thanks