change to posi - NastyZ28.com
Read this, post number 2. This is for the 8.5 10 bolt. The only difference is that the 12 bolt has 3/8 ring gear bolts with right hand threads and the torque on those bolts is 40-45 foot pounds.
All you need is a new Eaton Posi, a set of bearings and a super shim set. You need to know what your gear ratio is before you buy anything.