If your truck still has the SPID sticker in the glove box it will have a code starting with G. Let us know what is says and we can tell you. I'm too lazy to list them all. It would be a good idea to verify the vin on the tag against the one on the dash as a lot of older trucks have had the glove box changed. There is also the possibility the rear end has been changed sometime in the last 30 years unless the truck is fairly original.
If there's no tag in the glove box you'll have to pull the cover off the differential to check the tooth count. Somewhere on the ring gear will be a number like 41-11. Which would be 3.73 ratio.
There are generally few ratios for these trucks. 3.73 and 3.42 are the most common.
4.10 is rare, 3.23 is also really rare. I've never heard of a 2.xx ratio in an S10, but someone mentioned a 3.08 ratio in a Blazer with a 4.3 and auto trans. Never seen one of those though.
Probably the best for all around driving is the 3.42 gear. I've bought several with the G80 option (crappy factory posi) from Upulls. They are generally found in 98 to 05 Blazers, and another bonus is the factory disc brake rear end assembly with a rear sway bar.
There isn't a big difference from 3.73 to 3.42. The three trucks I've swapped from 3.73 to 3.42 are better on the highway (a little) and not much difference in town.
Even without a lift, it's not a super big job to swap an entire axle assembly. All are direct bolt in swaps.
I've done this to a 2.5/5 speed, and (2) 2.8 V6/5 speed trux.
Weird as it may seem... the 2.5 felt better overall. It cruises highway speeds better.
Hope this helps...
Confucius say: Good gear make good truck...
Quick check would be to raise one wheel off the ground (non posi) mark the tire and the shaft. In neutral turn the wheel two revolutions while counting the shaft rotations. If it turns a little over 4 times it would be 4.11, turns 3 3/4 rotations a 3.73. If you have a tru posi lift both rear tires and rotate the tire (both should turn the same) and only rotate the wheel one revolution counting the shaft.