[john walker's workshop]

you need to mount your ring gear on the posi unit. new bolts and lock tabs. then your original bearings and shims are installed on the same side of the diff that they came off of. then assemble it with some white grease and check the contact pattern. ZF posi units tend to be a bolt in, because their dimensions are pretty much the same as the non-posi diff. i'm sure there will be some discussion about that, but that's my experience. the one in my car has been quiet for 5 years using that procedure. and you can do the job with the trans in the car too.