the third is the complete drop out or carrier housing (everything that bolts into the axle as an assembly, make sense?)
the carrier is part of the third member that the crown wheel or ring gear is bolted to, it is held into the carrier housing w/ 4 bolts and 2 bearing cap retainers.
dont waste you time w/ trying to source a used unit, you'll spend way more than you need to. simply remove your axle shafts and drive shaft, and pull the third out. put third in a 5 gallon Home Depot "homer" bucket and either ship to GBR for overhaul or send to a local drivetrain shop like Sixstates, a rebearing at that point won't cost you any where near as much as used unit you have no history on. these diffs are easier than a ford 9" so any shop that can to those can do these. just tell them the backlash is nominal (.004"-.006") and the preload is about 20-25 inch pounds.