$500 for a used, as-is axle seems a bit steep. Geography may have something to do with that. We get plenty of salt in the upper midwest as well, and lately they have been also using lime chips as traction aid, just to make corrosion protection even more of a challenge. Many of the roads have had their first applications of "hot salt" (liquified) in late October, and that isn't likely to end until April.
Keep in mind that any axle from a '77 to '96 B-body should bolt in. Changing from a 7½ or 7-5/8" 10 bolt to the 8½" 10 bolt will require a shorter driveshaft (by about an inch) so buying the axle and shaft from a donor car may be advisable. Wagon axles will bolt in, but they are wider, so you may want to avoid those for a sedan unless you want to play with different wheel backspacing from front to rear (like my F-body).
If you get an Auburn style unit (cone clutches), the carrier will be important. These were more common in those earlier years, but could be found in F-bodies and B-bodies up to and through the mid-1990s. These will wear out simply from normal street use, so any used Auburn axle will likely need repair or adjustment, or at least a thorough inspection. It will also require a friction modifier additive in the oil or it will be noisy on turns and wear out even faster. The additive must meet the GM10582358 specification. CRC Sta-Lube (p/n SL2411) works for me.
If you get a plate clutch type (commonly called Eaton or Detroit locker), the plates, springs, and spacers may be more universal. A worn clutch pack can be repaired more easily than a cone type. These also wear out from normal use, and also require the friction modifier additive in the gear oil. These are more likely to be found in 12-bolt axles found in light trucks. These will also lock the two axles together at low vehicle speeds, much like the Saginaw gov-lock differential.
If you get a "gov-lock" unit it might not matter. The gov lock is a centrifugal locking pawl which pins the two axles together at a predetermined differential RPM, and they do not engage softly. They don't usually wear out, and instead they just break if hit too hard with torque. The gov-lock also does not require a friction modifier additive in the gear oil, whereas the other two types do. For normal daily driving this type is fairly reliable. They will lock the axles when traction conditions mandate it. They are not suitable for sport-type use since they do not withstand severe torque spikes very well, so if you are going to race or do burnouts in the Cappy this might not last. If you simply want traction in slippery conditions, this is a good alternative. They are commonly found in light trucks and vans, and on many sedans.
A fourth type is the torque sensing (Torson) differential. It uses helical gears to distribute driving force to the axles instead of clutches. These were used on many F-bodies from the mid-1990s onward. They require no friction modifier and essentially have no wear parts.