This would be a transfer case issue, it sounds like the detent is not fully engaging or you have lost the front drive planetary. Drain some of the fluid from the transfer case to see if there is metal shavings, also check the shifter to make sure it moving all the way.
Repair Safety Notice: This information is for general instructional purposes only. Vehicle repair can be dangerous. Verify all information, follow manufacturer service procedures, use proper tools and safety equipment, and consult a qualified repair shop when needed.