The bearings alone from Nissan will cost you close to $200. Your best bet is to find a diff out of a 300ZX - fairly recent model. I got one out of an 86 for my 78 because the guy was going to charge me close to $300 labor just to put $250+ of bearings in. I went to the local junkyard and got the one out of the 86 for $100, put a new front, and 2 side seals in and popped it right in. Works like a champ - 3.9 and quiet as can be. Seals cost a total of $24.95 and I used the Mobil synthetic gear oil as well.
R200's are pretty tough. On a stock car, they just don't wear out If its not leaking, don't replace anything (if it ain't broke, don't fix it). I would just change the oil and bolt it up and see how it works. I plan to do the same thing with a 3.9 I got out of an '82 ZX.