So, I have a 1985 Chevy S10 Blazer 2.8L 4-Speed Auto 4x4 and a 1992 Chevy S10 Extended Cab 2.8L 5-Speed Manual 4x2.

Now, which one has the higher differential gearing? If I need to get the VIN or something to be more accurate, then I can do that. I am not sure if these vehicles had a higher differential option at purchase.

 

Not sure about them just because I have little experience on the older ones, but the RPO codes in the glove box should list the rear axle gearing. If the sticker/glove box is gone, a dealer can look up the VIN and tell you what the RPOs were. They're probably pretty close I would think