This might be a very basic question for someone whose familiar with stock 8-3/4 inch rear diffs, but I've never been in this territory before...

The stock diff on my car is making a "RRRRRRR", growling sound when driving it.
The sound level is definitely louder than last summer, which triggered my concern this year.
The sound level goes down when I brake.
The sound follows the speed of the roadwheels, not the driveshaft..
The diff case isn't TOO hot to touch after a hard drive, but it's definitely warm.

I restored the car a couple years ago. Never opened the diff, but I did replace the case oil with factory spec oil up to the level of the plug. I have not noticed any leaks from it, but I haven't checked the current oil level.

I've got a stock '65 Satellite big block, the rear diff is the stock "open" 2.76 ratio (standard on an A/C car).

What do you guys think? (I'm a bit embarrassed I haven't even checked the case oil level yet or the FSM troubleshooting section..

I've never had a Mopar diff "fail" before. My old '66 Dart slant six had a similar "growl", but it never got worse and stayed the same for 200,000 miles. I was going to ignore this one too, but it's def getting worse over time and no apparent oil leak.

I doubt my skills to rebuild and reinstall the pumpkin. What kind of shop would do a rebuild? I have a couple performance shops around here. Or is it a machine shop I'm looking for?

Thanks for your input,
- Art