I assume you're looking for a complete axle replacement? since swapping only the gears requires a pretty high level of skill.

If he's got the 3.1GM engine, the rear axle is a GM corp 10-bolt (not to be confused with the Isuzu corp 10/12 bolt). Being a GM made axle, its actually pretty common and parts of readily available as GM used the same axle under many different vehicles.

if you don't know about the site www.car-parts.com, definitely check that out and you can source a replacement junkyard axle for relatively cheap.

-Rob

PS... what's shot about the gears? did he grenade them? gears don't typically "wear out" if installed correctly... if there is indeed failure, its usually pretty catostraphic.