I purchased a 12 bolt, non-posi with 3:31 gears. Going to put in a posi unit that was recently purchased. I am going to replace all of the bearings, even the pinion bearings. My question is by just replacing the bearings and differential, will the shim thats under the pinion still give me the correct gear pattern ? Also, What should the backlash be set at ?

you should check the pattern and backlash before taking it apart. take pic's. when putting in new bearings, things can change. the original shim is the best place to start, but you may need to adjust it up or down still. that is where the prior measurements come in handy. tom

I agree. Here are some instructions on changing the posi. This is for the 8.5 rear end, but it will be the same for the 12 bolt except the ring gear bolts will be 3/8 with right hand threads and the torque on them will be 40 insteead of 65 foot pounds. http://www.nastyz28.com/forum/showthread.php?t=19773&highlight=install+posi

if the rear end is in good shape now, why not install a power trax lock rite? Thats what I did, I never set up a rear end and didn't really want to learn the hard way. Jim