So, the following weekend, I pulled the differential pan to take a look. The gear oil smelled like burnt popcorn, but didn't seem to have any chunks of metal. It did, however, have that beautiful silver dust look that one can expect to see in the oil upon the initial break-in of an engine.
Beautiful as it may be, this was concerning--I was draining oil out of a differential, not a new engine. And as the differential has 120k+ miles, it should be well past the break-in stage.
After properly restraining the vehicle, lifting and supporting the rear axle, and putting the transmission in neutral, I started rolling the ring gear by hand.
Surprise #1:
Surprise #4:
The ring gear had four damaged teeth, but the damage on the ring gear was nothing compared to what I found when I removed the pinion gear:
Almost one entire tooth missing, broken out well beyond the depth of the tooth itself. All of the teeth were damaged to some degree, from pitting to large chips. Funny enough, the bearings--which I suspected were the cause of the noise--are in pretty good shape.
The solution to this problem is the low-mileage axle assembly sitting out in the garage, currently awaiting a new crush sleeve before installation. Here's to a happy installation and a quieter drive!
