what's the build date? look inside the driver's side door frame for the metal plate. it should have month/year like 09/06 stamped near the top.
if its before 02/07, then you have the improperly treated ring gear in the rear differential. Not a concern at all if you don't go offroad. But if you are in 4lo and hit some loose terrain, spin your tires and then catch traction, then one day you'll fracture it, loose a couple teeth or similar badness.
i know. Toyota warrantied it and replaced it with newer properly heat treated set. It's never been officially documented and there is no TSB about it, but if you find a good dealer, they will fix it and tell you that they are doing you a favor "just this one time".