from what I can tell just about any first gen loaded Blazer 4x4 seems to have posi.
Mine left the factory with one and the spare parts one that I basically bought had 1 in it. My bodyshop owner friend took just the posi unit out just before we made the deal. He left the gears in there with his old open dif carrier
My mechanic friend easily installed it in my bodymans first gen S-10 pickup 1 night
I don't understand how to change everything but my mechanic friend said that once I have the cover off it would all make sense
You should see some of the places that my Blazer goes without shifting into 4wd. And I don't think that I ever really had to.
Plus it steers nicely both on and off road.
Chevy did a nice job designing these units