Pinion Seal Replacement ?'s

Hi guys I was replacing my fuel pump this weekend(81000 mile crazy I know) because my fuel sending unit was junk being inaccurate and noticed my pinion seal was leaking pretty bad.
I need to know how to tell witch seal I have to get one on order before I take mine apart. When I call the auto store they tell me I need to know what size ring gear I have so here lies my question how do I tell witch one my Blazer is equipped with.
It appears all seals are special order!
2000 S10 Blazer 81,000 mile
4.3 W auto 4 dr 4x4
Last 8 on vin Y2248911

There are 3 different rears on a 2000 Blazer, but only 2 of them are found on 4wd models. You can't tell which one you have by the VIN. There is a code stamped directly onto the rear (I believe it is on the top of the driver's side axle housing, just left of the differential housing) or stamped onto a tag in the same general area. It is very hard to find and see -- you may need to use a mirror, and it may be obscured by dirt and rust. The fourth digit of this code is either a "C" or a "K" -- this letter indicates which rear you have (they were manufactured at different GM factories). If I were you, I'd get the seals at a Chevy dealer -- they don't cost very much, they should have them in stock, and I have found many aftermarket seals to be crappy. The easiest way to do it is to buy both the "C" and "K" seals, and return the one that doesn't fit. Then you'll also know which rear you have -- write it down because you'll need that info if you ever need to replace the axle seals.

As said you would have to code it out.
If you are not sure and can not code it out.
If only two dfferent ones pick up both and take the one you do not need back.

But remember the preload has to be put back to specs.
When replacing the seal.
Check preload before starting and make sure it is right when done.
Also at 81K I would be looking at why the pinion seal failed.
Is it lose? Grass wraped around it? Grove worn in yoke? Rear end vent stopper up? Bad bearings.
Pull inspection plate and check bearings and for metal shavings.
With inspection plate off you can get rear end info off of ring gear.
Gear ratio and sometimes ring gear sise.

Get a good repair manual for help on preload.
This link may help.
http://www.differentials.com/install.html