These machines don't need a "posi" in the rear, they have a fully locked rear end. There is no differential, it is a "spool" design. The front diff can be locked on certain models, and there's an aftermarket front end locker available from 501 parts for those that don't.
Just for the fact I bought the cheap unit. I also was not sure what the lock out button on the other units did..Was going to post on this as well. Was thinking the same thing and wondering why the op assumed he had an open rearend.