83 gp posi diff?

I have a 1983 pontiac grand prix with the limited slip diff and i would like to put a posi diff onto her. i was wondering what other kinds of cars could i take a diff off of. would one work of something like a regal or a caddy? they are all gm rwd cars with coil springs on the back end. i know that i cannot take one off an older car where there is leafsprings used, but how does that work with gm cars with coilsprings.
i appreciate any information anyone can give on the subject. what would work? what are the differences? what would i have to do to make something else work?
thank you,
George Komzak