3/4 ton rear end noise

[Repo_agent]

I have a 94 Chevy 3/4 ton. Its a 5 speed. I noticed a humming type noise (not so much whining) coming from rear. (9.5" ring gear.) It is noticable while cruising in 3rd or 4th gear and under medium acceleration in same gears. Sometimes in Odrive but not always. As soon as I let of gas it goes away. I changed fluid to 80w-90 and i also added a bottle of LUCAS oil stabilizer. I think after fluid change it got a little louder. It could just be me paying closer attention to it. Either way it didnt get better. Many people swear by Lucas oils.
Im not familiar with rear end noise. This sounds like more like a medium pitch eerrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr. Not really sure how to explain it on here. I always been lucky with my Chevys so I havent had much experieince with rear ends.
My truck does have close to 200k. I could tell the rear dif cover was taken off before but no telling what someone did. The magnet that was inside to catch metal fragments and such didnt have a whole lot of debree or metal on it.

I plan on keeping the truck and dont want to shop for a used rear axle just yet. So just looking for opinions.

If I remember right I dont hear it after 70mph.

[zirb]

maybe a broken belt in one of tires.

[Repo_agent]

Brand new tires. Yokohama Geolander AT+II tires. I recommend them highly.
Rides to good for that and shows no characteristics of a slipped or broken belt.

[maxwedge]

Rear end whine/hum could be worn or improperly meshing ring and pinion, or a bad pinion bearing, this would have to be taken apart and checked by someone who has experience in differential overhauls and diagnosis.

[mbumgua]

When you changed the oil did you inspect the condition of the spiders and side gears? I have seen the 9.5 spiders start to get roughness on the wearing surface at around 160,000 and cause noise. If you didn't check it pull it back apart and inspect these.