Hi I put a link in the other thread for diff gears.
As its second hand I don't think you can complain too much. You do not know how much the previous owner used them or whether he shimmed them after rebuilds etc.
Diffs should last quite a while but are still 'wear and repair' items. They are part of the transmission and should be checked and rebuilt with new gears/ componenants where necessary, as well as having the fluids changed, regularly.
Shim them correctly though and they should last a long time.