Well, we were too late. Son drove home the other day complaining of chattering sounds on turns. I took it out for a brief test drive and it was clunking loudly from the rear axle at anything other than a <5mph crawl, and moreso when turning. Got it in the driveway, opened up the diff and it there were no obvious broken teeth, but there were small metal pieces on the bottom of the diff after it was drained. Biggest pieces were about the size of a grain of rice. I cleaned it out as best as I could, put a new gasket on, closed it up, filled with ST 75W-90, crossed my fingers, and backed it out of the driveway. Still clunking. 
Parked it out on the curb outside our house and going to have it towed to a mechanic.
Parked it out on the curb outside our house and going to have it towed to a mechanic.