Vehicle starts to shake when I hit 50-55 mph

Have you recently changed two of your 4 tires? I suspect this issue is in the transfer case and not the transmission. You have either the NV140 or NV245 transfer case. These are very different in their operation but are the same in the fact that the vehicle is basically in 4WD all the time.

Because of this, the tire circumference of all 4 tires has to be equal. If it is not then basically the tires will spin at different speeds and the transfer case will vibrate.

If we just changed tires then we need to change the other two to match and retest or we may have to change all four. Please let me know and we can go from there. We can try adding some friction modifier to the T-case but that is only after we fix the tires.

Okay. This can still be the transfer case so I would suggest just adding this friction modifier. This will allow the clutches to slip a little better which should cut down or eliminate the vibration if I am correct. I attached a picture of what you need to use.