When I put my Tahoe in reverse and give it gas it locks up and makes a grinding noise it sounds like it's coming from my transfer case... anyone know what could cause this?

 

It sounds like you're not making a total shift into 4WD. Check adjustment of operating mechanism, or, disconnect from below, and try to shift it manually from under your ride. When up on a chassis rack, you can rotate the wheels to make it easier and see if it actually is engaged.