8 1/4 axle inspection.

I inherited a 3.55 decades and decades ago that whined when I got it. A customer of mine wanted one so I took him for a ride with mine; I was swapping it out anyway. Yeah hesaid, that would be OK, so we struck a deal, and he paid me to swap it into his car, a very sweet 72 Challenger. The deal was if he didn't like it, I'd buy it back, and he'd only be out the swapping labor.
Well, in his E-body, it was pretty noisey, so Hyup, I still own it. I think That diff came in my 69 Cuda that I bought in 1983ish. It's not half as noisey in my car. Over the years I have worked on the pattern and changed bearings etc, and nothing changes.

The point is, if it ain't putting metal-dust into the oil, they might go for decades and decades.My 3.55s must be over 40 years old and have always had a slight whine. That 69 was born with 3.55s
I recommend to pull a sample of hot oil out and check it for metal. If it's clean, put it back and drive it. But if it makes pretty swirling patterns when you stir it, well she may be on her way out.