I wheeled the crap out of my jeep with stock axles and 33s and they held up fine. They weren't locked though. Even though they were ok for me, other people have had problems with smaller tires.
I've moved up to 35s and a Ford 8.8 since then, and I definitely feel relieved that I don't have to worry as much about breaking the rear axle.
92 YJ, 4.0, D44/60, 5" BDS springs rear, SOA front, 1" booms, 1" BL, 35x12.50x15 MTZ's, 4.10 gears. It's never done though.