What the heck is up with 8-3/4" rearends?

The thing is, if you are building a performance Mopar, it is pretty much a given that you need an 8-3/4 (ignoring the Pro Street guys and their Danas). But probably less than 20% of A-bodies came with these rear ends. For everyone else who wants one, the only place it can come from is some other guy's car. So you have to make it worth his while. The Sure-Grip ones are also pretty expensive to rebuild, so that factors into the price.