Richmond Gears makes aftermarket Rings and Pins for the O-type posi unit.
http://www.richmondgear.com/01gm.html
They only fit for the #671 (3.08's 3.23's) posi carrier though.
This information is from Brian Trick (as mentioned above). I've mentioned this before. If your going the posi route, I highly recommend for anyone to talk with him. He is awesome.
By the way, Brian can make those richmond gears fit #672, #588, and #673 posi carriers.