I drove with a rear end whine on acceleration and certain higher rpm's for a few years. I changed the diff fluid a couple of times and helped the noise a little bit. I bought a junk yard axle and took my time welding it up and replacing bearings, new brake lines, painting until I had the time to swap it out. When the noise got bad enough so I could hear it above the radio, I figured it was time. I took out the old axle and the ring and pinion where worn somewhat irregular and there was one tooth on the ring gear that was chipped although I never found pieces in the diff fluid when I changed fluid. The sound was there when I bought the Jeep so I figured the damage was already done before I bought it cheap because of the whine.
Junk yard Dana 35 axle was about $200. It's been good for a few years now. I never changed out the ring and pinion on the replacement axle, I just put some colored grease on the ring and pinion to see it was making a nice pattern. I am not a gear expert like some here so I didn't want to mess with setting new pinion depth etc.. I guess I am cheap.
The only problem I had was the guy helping me cut the axle out of the donor vehicle with a torch cut the abs wires which I had to fab up some replacements. I always forget to take my time in the junk yard and remove all lines and wiring harnesses in tact. I have made the same mistake several times in the junk yard, wanting to get the part out quickly.
94 ZJ,4.0, AX15, NP231