Broken axle maybe?
Sounds like a broken axle to me. 2wd no forward motion. 4wd it moves. GM uses "open differentials" that provide equal torque to each wheel. (on the same diff) That means that if zero torque is applied to the left wheel, then zero torque is applied to the right wheel. If you have a broken axle, you get zero traction at the other wheel. See more at
How Stuff Works
Jack up the rear of the burb so that both rear wheels are off the ground. Spinning one wheel should turn the other wheel. If it doesn't, its likely a broken axle. If you don't have a floor jack to lift the rear at the differential, take it to your mechanic and put it up on a lift.
Open diffs are not the best for serious off road stuff. I would rather go with a locker differential. The air actuated lockers are my choice.