So, I was kinda surprised to see the axle break where it did. I was trying to figure out why. My first guess was a wheel bearing went bad, and I couldn't hear it over the tires, even though they really aren't that loud. The outer bearings all looked fine as did the bearing surface. When I dug further, got the brake hardware separated from axle housing, I saw the axle bearing surface looked burnt, RIGHT where the axle broke, but the bearings themselves look ok. That seems weird.
You can kinda see the bearing surface, it has the dark-bluish tint to it.
So... I had noticed my axle housing had a wet spot half way between the pumpkin and the wheel the other day, but only got to look into it today. It seemed to have a bunch of crud built up on it, and as I tried to scrape it away, I found out that crud was actually metal. The housing has been welded...poorly.
So... my guess is that one of the previous owners somehow managed to crack the housing and had it welded back together. Continuing my speculation, I think the housing is bent ever so slightly, putting stress on the axle bearing, heating up the axle over and over and eventually causing it to fail.
Whatever the cause, it looks like I need an entire rear axle assembly to fix this. Honestly, that's a bit relieving. I know I can do that with the tools I have at my house, no bearings to mess with (although replacing them wouldn't be a bad idea). I am going call some junk yards tomorrow. I am also very very grateful this didn't happen when my wife was driving 70 mph on the freeway. It would have been catastrophic.
Thanks everyone for your help and insight. Looks like I might have a spare locker for sale here soon.