Does anyone know if the transmission mount goes bad that it could cause vibration in the rear around at 70 to 80 miles per hour or could it be tires out of round because I had my tires balanced twice and they still shake me out the truck

 

You can perfectly balance a square tire. But it's not going to roll smoothly, no matter how many times you balance it. Could be a bad tire, or a bent wheel, although tire issues usually show up around 45mph and above. 70-80 mph vibration, could be a universal joint, or a problem with the driveshaft being out of balance, or bent, etc.

 

I installed new u-joints but very little change. I honestly think that the tires are the problem

 

Thanks Captain Hook, by the way the transmission mount looks shot it's dry rotted I'm going to change it next

 

Poorboy old school check - jack each wheel up one at a time and for safety support vehicle on a stand not just a jack.

Place a block of wood ECT on the ground almost touching the face of the tire.

Rotate the tire slowly by hand and watch for rubbing on the block, if the block stops you from rotating the tire move it back a little, repeat until the tire spins freely.

Rotate the tire and the furthest gap between the block and tire is the amount out of round in either tire or the rim

You can do the same on the side of the tire to check for rim wobble due to bent rim

 

Thanks, I'll let you know what I find