Jamie, it's not as much that they are rare, but just hard to get and expensive. Since most of the hot rodders use them they are starting to get harder to find (eventually they will be rare). The prices are going up on them due to that fact. If you do in fact have 4 of them I suggest hanging on to them. It's like top loader 4 speeds. They aren't necessarily rare but are getting very expensive if you want one. I don't even run 4 speeds but I have 4 of them. 5-10 years ago they were bringing $3-400 bucks. Now they'll bring $600 and up. Who knows what they'll bring in another 5-10 years. The same thing applies to your 9" rear ends...could be gold if you hang on to them long enough.
9" rears are not particularly rare... the right 9" rear for a Mav is always rare. Has to do with our cars and it's very narrow rear and small tubes. Dave