Here's the pics. It's hard to see in the pic, but it is broken all the way through, and the spider gear axle was the only thing holding it together. There was some minor spider/carrier gear wear which I'm pretty sure happened after it broke.
The shop did end up putting in new ring and pinion gears too, because it was clear they have gotten very hot. But they have no wear, so I kept those as a spare set.
I still have no idea why this happened. I don't think it was a defect, since it did make it 150,000 miles before it broke. But I do suspect the V10+Banks+Predator+4.56 gears didn't help any. Which means before I add any more power, I probably need to jump up to the Dana 80 with 11.25" gears.