I have seen a complete A body unit for $150 before. They don't get the spotlight like the 8 3/4 and Dana 60, so they go for cheap. I'm sure more than a few have gotten sold for scrap over the years.
I saw an 8 1/4 rear 4.10 ring and pinion for sale here yesterday for $100, I'd check Craigslist first for an 8 1/4 because shipping one would not be cheap, or you could wait for one to show up for sale on here and hope it's within driving distance. One thing to note about the 8 1/4 is that there are two different carriers. 2.71 and down used one carrier, anything higher used another. One is 27 spline and one is 29 spline, if I remember correctly. Just something to think about when you find one you might buy or you need to buy parts like a sure grip.
Somewhere on the rear end there should be a tag with the gear ratio on it, assuming nobody has messed with it over the years which is entirely possible.