I believe it just went by year assuming this is the original rear end. I know I was doing a complete bearing swap for a friend of mine (axle, carrier and pinion bearings) on his 98 and when I picked up the bearings, some of them didn't fit. Found out (talked to the previous owner of the truck) the rear came from a '92 which had different sizes. Why there would be two sizes seems strange, I never recall that before. Just yank the old one and bring it with you. Too easy.
Could be wrong though, that's just my personal (limited) experience. That's when I learned to pick up both sizes if you don't want to pull the part first, especially if you're just doing a couple bearings, when you have to get two whole boxes of stuff it's a bit of a PITA but I live around 40 minutes from anything but a gas station, making runs into town all the time sucks.
Also rent, borrow, buy or steal a slide hammer (in case you didn't know). I have done them with a pry bar and a lot of cuss words, and I assure you, it's not worth f***ing with. It can be done but...why? Trust me on that one.
Good luck, sorry I couldn't be of more help.
*Edited for elaboration