FIXED!!!

It was the slip yoke that was causing the problem. Denny's driveshaft sent me the wrong slip yoke (too short)! At highway speeds when I let off the gas the yoke would try to come out causing a bad vibration. Also the driveshaft was a little out of balance from when I had to burn the factory U-joint out with a torch. I got a new slip yoke and the driveshaft balanced today and it runs A LOT smoother.