From what I hear, the ring gear in the M226 /w Locker is amazingly thin to allow room for the locker. This seems to be what's failing in the rears. Though, yours sounds more like you first blew the spider gears.

There was a guy on the Club Frontier board who has a 2wd Nismo and blew the rear (the NISMO frontier has the same rear as the OR X). Nissan would not cover it under warranty because he had blocks on the rear axle. From what I remember he took it to multiple dealers and got the same answer. He then went up the ladder and got no help there.

My rear has been fine and I have changed the fluid three times now (due to peeled back cover) and never found any unusual wear or exessive metal particles.

On a side note. The M226 rear is also used in the Titan with a lot more HP than the X or Frontier and it holds up pretty well. So I don't think the rear is all that weak. You can't go by XterraRacer's luck as he is beating that truck as hard as possible.