My shaking issue has been fixed so I will tell you what fixed mine and I would almost guarantee this will fix them all.
I was building up my 6l80 with some performance parts and sent my shaft out to a local shop to check balance, he said balance was perfect but it was out of true...meaning when he chucked it in his lathe it had 1/16 of an inch of runout in the center of the shaft. It was so bad you could obviously see it when spinning so he trued it and now my truck runs as smooth as glass with no shake, no shimmy, no vibration, nothing, it's never driven this good ever. So take your driveshaft out and have it checked. BTW it only cost me $86.00
RT