I'd put my money on axle-shaft. If it was diff you'd hear all kinds of noises. Put transmission in park. Jack each side separately. If you can spin a wheel while other on the ground and prop shaft isn't sniping it's that axle shaft. Pay attention to prop shaft. Yes Monty axle is really tough but I've heard of this happening. You never know what kind of abuse happened before.
If you can lock a diff and get going. That's also a good indicator. I made it 200 miles home this way after I broke mine.
1992 Montero - overland eqpt. (SOLD)
2002 Montero XLS - died protecting the master.
1997 TLC 80 - (SOLD).
2008 Dodge Power Wagon, 35's, FWC Ranger.