For your differential, look around for a local 4X4 shop that services the off-road crowd. If they truly support the off-roader's then they will be well versed on swapping out ring & pinion, and also dropping in a posi-trac. I'd strongly recommend that you consider something like an Eaton/Detroit Tru-Track spiral gear locker, which is very, very street friendly, locks solid, and is all but bullet proof.