Is it correct that on a rear end that has posi that if you jack both wheels off the ground they should both spin the same direction when spun? Also can a rearend be converted easily to a posi?

If it is a posi and you turn one wheel forward the other wheel will also turn forward. If it is not a posi or the posi is faulty the other wheel will turn in the opposite direction.

A differential can be converted to a posi.. it would require a posi carrier assy.

What would it cost to convert a 12 bolt to a posi and is it difficult to do?

A Eaton posi for the 12 bolt is $350. It requires setting the backlash on the ring gear and getting the bearing preload right. This is done with shims. When you buy the posi you should also get a good shim set and new bearings. The super shim set is $22 and a new set of Timken bearings is $40. You will need a dial indicator with a magnetic base to check the backlash and you will need a micrometer to measure the shim thickness.