I have a 96 4.0 with 195K miles, 35C rear axle, 3.73 tow group gearing, and it's non-LSD. Last fall I replaced the outer leaking bearing seals and installed new Timken outer axle bearings. I also cleaned the differential out and filled with new Royal Purple 85W-140 synthetic gear oil w/friction modifier. This probably wasn't necessary to add this type gear oil since I don't tow anything anymore and always used regular Valvoline 80W-90 (spec) before. Believe it was Foundrydude who also mentioned friction modifier gear oils may cause additional bearing wear.
When driving on the freeway at 65mph I can hear kind of a high pitched whine noise coming from the rear end. Once I let off of the gas and coast in drive the noise stops. There's no vibration in the drivetrain at all. Believe this noise was there even before I installed new outer wheel bearings and replacing them didn't cure it. But the drivers side rear outer bearing was bad/worn.
I have a very slight leak in the pinion seal but don't really have the tools to remove the yoke/nut but I'm sure I could probably get some loaner ones. I'm a little leery of tearing this apart and getting the preload just right in tightening with the old or a new nut.
Since my wife's TJ is broke down she's driving my ZJ 50 miles round-trip to work every day. In a couple weeks I'm going on a 250 mile trip to Cinci where my son's running in the marathon there. I don't need any problems with the rear end on this trip but it should be noted the noise isn't getting any worse but just there.
While it most likely won't stop the noise I was thinking about draining the rear end and putting new regular 80W90 gear oil back in. Maybe a waste of time and money?
I'd appreciate any input on what this high-pitched whining noise could be? I don't think it's a gear noise and more likely the pinion bearing or worn inner axle bearings. Thanks!
Below is info on gear-bearing noise from the 96 FSM and not sure if the noise I'm hearing is listed here? The 1st part is related to gear noise.
No, I never did figure it out or fix it. Noise has stayed the same for months with my wife driving it 50 miles round-trip for work M-F. So If the noise gets worse, or the leak, I'll do something about it but not for now. I'd go ahead and change your fluid then see what it does or sounds like. If you find metal in the bottom of the differential that's a different story but mine was clean. Good luck.
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.