last night i went to take off in my jeep when i heard a loud bang and the jeep wouldnt move, it was running fine but wouldnt move. i figured my diff had failed so i put it in 4wd and it took off fine. i went out this morning, put the jeep in 2wd and it drove just fine with no more noise than it usually makes. what could it have been? maybe it popped out of gear and i didnt notice because i put it in neutral to shift to 4wd? maybe i never even engaged first all the way?