'84 ks leaky differential

[Andrew Pawlisz <apawlisz@emerald.tufts.edu>]

Hi,  I have a 1984 4000s with a recurring leaky differential problem.  About three years ago, I had to replace the original differential as it busted on me for the lack of oil (the owner's manual does not mention to check the differential oil: supposedly maintenance-free).  Well, the new "used" differential is leaking again, but I know better now to top-up the oil now and then.  The problem is that I have to crawl under the car every month or so in all kinds of weather (no garage) and the oil drips onto the exhaust producing a stench that spoils the otherwise enjoyable ride.  Still remembering the horror of the last bill for the differential ($1500), I dread a trip to a mechanic.  I went for an estimate once to AMCO, but they want to charge me $60 just to look at it.  "What is there to look at? It is leaking, and that is all to it!  Can you fix it? - Well, no $60 no estimate."
My question is that do I have to replace the differential again with another used unit (will leak again soon thereafter?), another remanufactured unit (new seals and new bearings?), or just replace the offending seals on the unit that I already have?  What would be the approximate cost of each option and who might be the best choice for this job? I plan to keep the car until I find a '87 4Kcsq.

Thanks 
Andrew

1984 4000s - 
1987 4000csq - looking for one