It looks like there is a simple explanation for the gear oil all over my truck. I overfilled it when I replaced the oil seal. And evidently the excess backed up the vent hose. It dumps right above the driveshaft, which slung the oil everywhere.
Sounds plausible. So that is the story I am sticking with.
I opened the rear differential and everything looks fine inside so that leads me to believe the vent theory.
Thanks to Racecar Man for enlightening me.