the rear in my 64 c10 when i got it had already been swapped as they did a 5 lug swap. its a 12 bolt and i was told a 3.73 and its from a suburban, but he didnt remember what year or gen. anyhow, i havent confirmed the gear ratio yet, but i have just bought a 66 for parts with a good cab, fenders, hood, etc.. to swap on. what i need to know is are posi units common from factory for these rears? if not what trucks should i look for? or is it just going to be a crap shoot at the junk yard to find one? are there any differences i need to worry about finding a factory lsd unit? i know with the 10 bolts theres 8.2in and 8.5in. any issues like that with this one? i know my truck is a series 4 carrier and the car ones were series 3 so car diffs are out, but dont know if there could be any other snags to run in to. im tired of it only spinning one wheel when i floor it.

 

no one here is knowledgeable about the gm 12 bolts?