Before I started tearing into a rear diff I would try changing the lube and add some limited slip additive in with it. I prefer the factory GM brand p/n 1052358 , not the kind that you buy at your local parts store. I have a Auburn posi in my truck at it will raise holy hell without the additive in it. After you drain the old lube and are ready to refill it , put the additive in first this way it get mixed in better. Rebuilding and setting up a rear end is not the easiest thing to tackle but I believe you can get some pretty good manuals and instructional videos for some of the gear manufactures like Richmond. You will also find that it takes some special tools such as presses, pullers, and a dial indicators etc.... not everybody has those in their tool box. I usually take mine to a professional to have it set-up . Hope this helps.
