s10 fear end

Well, based on the motor size, and the fact that's2wd suggests that your rear end is the 8.5"- You can use your vin number at the dealer, they can bring up the OEM part replacement (at which point you'll get PN#)

3:42 is very common with gm but your Vin is definitely the right way to find out.

Yes- you are right 3.42 in the ring gear size for a manual trans (5spd)- however as an auto (4spd) they had a 3.73. Hey sackart check in the glove box and see if you find the letters GU6 (3.42) or GT4 (3.73)

As said before glove box codes will tell you ring gear size. The 7.5 rear was way more common in an s10 then the 8.5. If you look under the rear of the truck at the diff there will be "tabs" in the casting on the bottom. The 8.5 are square off tabs on each side and the 7.5 are just 2 points sticking down. Google (7.5 vs 8.5 gm diff)