I'm running a 6" lift on 35's, and whenever I get up to 60 mph and let off the gas, the truck will kind of shutter/vibrate. But it only does this from 60 down to about 53 mph. Its not violent shutter, but you can tell that its there. I took it into one shop and they said they thought it is some kind of driveline problem. So I took it to an off road shop and he said that it is because of the angle of the driveshaft. He told me I have flat blocks for the rear lift, and if I was to put some angled blocks it would fix it no problem. Any thoughts?