someotherguy
+1y
Most likely it's a Dana 70, watch out, gonna have some TALLLLLL gears in it. Gearsets available for that rear would be 4.56, 4.88, 5.13, 5.86! If you're running tall tires (31" or better) it won't really matter, will give great take-off from a dead stop, but if you have short tires man you're gonna be spinning the shit out of your engine to go highway speed...especially if you have a non-overdrive transmission, you didn't mention what you're putting it in.
I'd say $300 for a strong dually rear with discs probably isn't a bad price, as long as it's going to work for what you need. Be sure the width is going to work out and find out what gearset is in it.
If it's a Dana rear (and it almost surely is) you can positively ID it from the bill of materials tag which can be in various spots depending on year and model; usually it's a metal tag attached to one of the cover bolts, but sometimes it's not a separate tag at all, and you're hunting on the axle tubes for stamped numbers that may be hard to read after all these years. Find the numbers and google up X510-9 and choose the link from Dana's site (usually the first search result) for the Dana/Spicer catalog, it also shows you different locations for the B.O.M. and has more info in it about Dana rears than you could ever imagine...
Richard