96, s10, 4.3L "x", 5 spd, rwd, ext cab

How do I figure out which pinion seal to use? Oreily has options depending on the size of my ring gear. 8.5", 7.5", 7.625" etc. Neither my Chilton or Haynes says anything I can find about different sizes. The parts store guy didn't know either. Any ideas.

Here's the oreily website link.

List: Search for 'pinion seal' | O'Reilly Auto Parts

 

a little more info: It's got 138 (i think) and a 96 stamped on the front passenger side of the dif. On the bottom lip it has 26041413. And it's got the 10 bolt cover.