You need to discover what your current rear differential is, then you can proceed forward with what you might be able to do for the Blazer. If I'm seeing things right, your rear end should be of the 10-bolt 7.5/7.625" variety (used in both the 4cyl/6cyl, 4x2/4x4 models). It can, however, utilize either the 26 or 28 spline axles.
You should be able figure out what you have by looking at the RPO codes. As long as the dash hasn't been changed out, you should be able to find these on a sticker located on the glove compartment door (open it up and there she be). If you are unsure about whether it is the original sticker, it should have the VIN stamped on it as well, which will you can check against the main tag. There are several pertinent to what you're asking about. Look for these:
GU1 AXLE,RR 2.41 RATIO
GU2 AXLE,RR 2.73 RATIO
GU3 AXLE.AR 2.93 RATIO
GU4 AXLE,RR 3.08 RATIO
GU5 AXLE,RR 3.23 RATIO
GU6 AXLE,RR 3.42 RATIO
(It should be one of these, but if you don't find a "GU" code, please post up all of the codes which start with "G" and we'll get it figured out.)
Just for confirmation sake, it should have a:
G89 7.5" Ring Gear
(If not there, the rest of the "G" codes should clue us.)
If you insert your VIN into this decoder website, you should be able to get your entire RPO code listing and what they are for your vehicle. You should be able to figure out exactly what your vehicle has in it and then you can start searching online for exactly what parts you are going to need to make the change.
Basically, you are going to have to change out the differential in the housing. You shouldn't, however, need to change out the ring/pinion gears as long as they are in good shape. They should bolt right up to your new differential, which is a good thing. When it gets changed out, the setup should already be there so you won't have to go through process of shimming and adjusting anything. You also shouldn't have to remove the pinion, for which the setup is a PITB (just rear ends in general are a PITB to setup). It takes a skilled/knowledgeable hand to do the work. It's not something you can read about in a book and tackle. If you've never done it, leave it to a professional.
When you do look for a replacement differential, you do not want a full locker. You'll want to look for a limited slip. This will allow you to use the vehicle easily, without fear of the differential causing binding when going around sharp corners.