Basically since I've been driving this (bought with 136k, now 168k) I've had the rear end "hum" -- sounds like off-road tires.
I don't know if it's gotten worse over the past few months, or it just seems louder because it bugs me every single mile.
Anyway -- at this mileage I thought it would be wise to replace the fluid. It didn't look good. It had a metallic shimmer to it and was a lot darker than I would liked to have seen.
I also noticed 75-90 stamped on the cover.
I added the 75-140 synthetic and a bottle of champion LSD modifier (I wanted motorcraft, but that's what Oreilly had).
I hoped the new fluid might reduce the noise that drives me crazy, but honestly didn't expect much change. I was happy to find that it's maybe 50% quieter. Quiet enough that I think I might even be able to forget it's there with the fam in the car.
I have a 2002 EXP and it had the noise from 30K miles onward. I changed the oil a couple times and added the friction modifier even tho it has no LS unit. That did seem to help 50% as you said. I drove it 120K miles that way. I finally tore it apart 3 months ago and replaced all the bearings. The pinion BEARING was burnt and excessive clearance. Many say it is gear noise and I was agreeing with that until I did the bearings. I think the factory got the bearings too tight and cause the problem. I have the same gears now, new bearings, and NO NOISE.