prg tranny mount causing highway vibration?

I have a vibration/shudder I can't figure out. I replaced my transmission mount with a prg and it does vibrate the cab at idle, but no big deal. However I have a bad vibration and shudder that starts around 50mph gets better around 75 and smooths out at 80mph.

I put new tires on around the same time so I figured it was the tires. They have been balanced 5 times, yes 5. And one tire was replaced as it was suspect. There really has been no change in the vibration. I notice it through the steering wheel and at some speeds even through the cab and can see my passenger seat shake. Can't really feel anything through the shifter though. I am at a loss. As anyone experience vibrations at hwy speeds from the prg trans mount? Any ideas what it could be?

2005 le 4x4. All fluids replaced. Rear ujoints replaced only to find out they really weren't bad. Front ujoints are newer and look good, just greased them. Had an alignment done after the tires when the ujoints were replaced. I'm stumped and kinda frustrated.