1989 s10 blazer rear differential small gears worn, teeth ok.

Should be a tag on the differential under one of the cover bolts that has that info on it. Otherwise you will have to take cover off and the ring gear will have the ration stamped on it on the edge... You could also count the teeth on the ring and pinion, divide the ring gear count by the pinion gear count.

Replacing gears and/or bearings in a differental is not an easy project. You'll need a press. The positioning of the gears requires a great deal of precision. Shims are used to acheve the correct setting. Preload setting on the pinion bearings is critical. I would recommend leaving this one to the experts.

Should be a metal tab bolted to rear inspection cover of differential displaying gear ratio or count the number of teeth on pinion (small gear connected to drive shaft) Divide number into the number of teeth on ring gear

The ring gear is mounted to the flywheel for your starter teeth to engage and turn the engine. Most all cars have this ring gear. If you are having problems starting it may be worn teeth at one point. Auto trans and manual trans will have a ring gear for the starter.

two ways this can be done.
1. in the gove box there is a build sheet a square
piece of paper thats glued on inside with a bunch
of three or four letter codes . call a dealer and they,
tell you which ones tell rearend info.
2. remove rearend cover , you will see veryis gears
count the teeth on the big gear on the left side,
write it down,count the teeth on the gear that comes from the drive shaft,write it down.
you can call dealer or divide the big gear[ring gear]
with the pinion gear the small gear that comes from
the drive shaft.

We can approach this one of two ways.
One: take the differential cover off of the axle and count the number of teeth on the ring gear; next count the number of teeth on the pinion; now divide the ring gear count by the pinion count to get your gear ratio.
OR
Two: Tell me what engine, transmission, and factory tire size your vehicle came with and I will look it up.

you can count the teeth on the ring gear and divide that # by the tooth count on the pinion. An example is: ring gear 100, pinion 10 ...100divided by 10 is 10, expressed as 10 to one. (fake #) your diff is likely a 3 series such as 3.23 to 1. or a four series such as 4.11 to 1. If you have four wheel drive it is critical to match exactly. if not you can get better performance by changing ratio to something that better fits your needs. 4 series pull loads well, 3 series are better on fuel.(depending upon terrain)

Manual or Automatic? The 1986 Jeep Cherokee 4x4 used both.

With the manual version, it would be best if you had someone who has done this a few times to help you.

1.You need to check all the gear teeth. No cracks, chips, or broken teeth. The reverse gear, and the countershaft gear that mates to it, can have slightly worn teeth, and maybe a tiny chip, on a few gear teeth. These can be ground down to a serviceable condition, if the chip is a very slight one. The reasoning behind this, is that the reverse gear should be engaged when fully stopped. There is no brass synchronizer ring.

2.The tapered part of the 1st, 2nd, 3rd, and 4th gear, should be checked for clearance, with the brass synchronizer ring pushed up against it. If it is too loose, you cannot use this ring. If the triangular teeth on the synchronizer ring, are worn too much, you cannot use this ring. These teeth are what engage to speed the gear up, so that it will match the rotating speed of the rest of mainshaft. Your transmission will also pop out of gear, if these teeth are bad.
(Synchronizer rings are also reffered to as 'Blocker Rings)

3.Check the detents in the hubs. The rectangular shaped pieces of metal, on the hub/s, that the sliding collar engage. Check the circular shaped metal springs for these detents, also.

4.There are probably 113 small needle bearings, for the countershaft. This are so much fun to remove and install, that you'll want to stay up at night and do it! One front mainshaft bearing, and one rear mainshaft bearing. The input shaft cover has a bushing.

Below are four links that may help explain the basics. Past this point, I suggest a detailed manual. Your average Chiltons and Haynes manual, will probably not have this info. You need a shop manual.
I have included automatic transmissions also. This is a job best left to a pro. Sometimes they make mistakes too!
1.http://www.howstuffworks.com/transmission.htm
2.http://auto.howstuffworks.com/automatic-transmission.htm
3.http://en.wikipedia.org/wiki/Manual_transmission
4.http://en.wikipedia.org/wiki/Automatic_transmission

Unless you are experienced in diff gear setup, this is not a job for a do-it yourselfer. You need a torque wrench , dial indicators and experience to set a diff up properly...If not, you will waste your money!!! Usually cheaper and easier to swap complete units, drum to drum...then all you need is to determine exact ratio of the front unit and match it. (ring gear teeth divided by pinion tooth count= ratio )
You may not like my answer, but I'm trying to save you time and aggravation and $.!!!!
good luck

The C/Tr: C refers to the colour of the vehicle Tr refers to the interior colour A/TM is your transmission type. None of those numbers identify the rear end ratio.

What you need to do: Since you have to replace the rear end, yank it out. Examine the ring & pinion gears on the differential carrier. Mark on each a starting point, and count the teeth on both the ring & pinion gears. Now, after you have both totals of teeth, divide the # of teeth on the pinion by the number of teeth on the ring gear. Example: 12 teeth on the pinion, 46 teeth on the ring
12/46 = 3.83 ratio