The S10 rear is limited in gear selection and posi option. They are also all leaf spring rears and more expensive in junk yards. I learned all the Coil rears are a dime a dozen for the Buicks ETC.. F body cars. The internals are identical and many are posi units. That being said if you want a 4:10 or 3:77 posi for your leaf rear get the cheap coli rear and install carrier and the 4 to 3 ratio something gears from the leaf for a cheap posi setup.
I did this in a 383 powered S10 pickup and have blown several rears so found a way to get cheap posi replacements. The GM parts guys will say they only have open 4:10 rears for an S10 truck in the books !!