If you put a pry bar under the transfer case and pry up does it move any? If so you might just need a new mount to hold it firmly.
If the t-case is willing to go in and out of gear and you can find all of the transmission gears at the appropriate RPM without grinding them it doesn't seem internal. Seems more like the t-case is trying to spin around with the driveshaft because it's not held down properly.

 

That is the complete drivetrain vibrating from the motor when you lug it down. The transfer case is not even spinning to vibrate.

 

so what was the resolution?