Welcome to the forum. There are lots of aftermarket places which offer the limited slip rear ends, or third members to convert which rear end you have. Do you know which rear end you have to start with? The 8.75 rear is much stronger than the 7.25, and a swap from any of the A body cars will fit, so 1965 to 1976 will work, and not sure if you have the large or small bolt pattern, either.
"Posi" is a GM marketing term. Chrysler's equivalent is "Sure Grip". As Dana44 mentioned, they're Limited Slip differentials. Auburn was a prominent manufacturer of limited slip differential carriers for Chrysler axles, and there's a ton of aftermarket support for the A-body, even if one may have to replace the entire axle.