Unfortunately, you are going to have to open it up. A rebuild is probably something you might be looking at.
EDIT: Well if the stick is moving when it's popping out of gear, it might be the shift rails. If it is not moving, it's probably bad gears. Grinding is never good with gears. You really need to determine if its actually in gear. A bent shifter or loose linkage could be preventing it from fully engaging in gear. If its not in gear completely and popping out and then making grinding noise, it could very well just be linkage. If its in gear, not popping out of gear, and just grinding, it's probably bad gears.