does he need a ring and pinion or a rear end? if his rear end is shot completly and needs rebuilt then when he gets another one it should have a good r&p in it. if he needs a R&P but the rest of the axle is good he needs to take it to the shop for repairs in which case the shop will provide it. from your question it would seem you have never done one of these before so it will be a complicated fix for you to perform on your own.
"If you pick up a starving dog and make him prosperous, he will not bite you;
that is the principal difference between a dog and a man."
-- Mark Twain