2002 Mitsubishi Montero Sport XLS transfer case issue.

In AWD it made whining and then a grinding noise. Stopped, drove in reverse for a few feet; heard a clunk, and was able to drive again. Later that same day, it did it again...no luck getting it to move in AWD or 4H (same whining/grinding noise); it moved (no noise) in 4L. Is there some sort of chain drive in there that could be slipping? I really hate to replace the whole transfer case if it's a fairly simple repair. Also, if anyone knows where I can get my hands on some sort of repair DIY manual for this vehicle, I'd really appreciate it. Thanks!