OK, so filling up the jeep last night with the preciously expensive go-juice.
Attempt to drive away, and the jeep doesn't want to move, then a sproing, and she rolls freely. I take a look below thinking a d/s u-joint locked up and then blew. But no.
A u-joint cap on the drivers front shaft blew it's cap off. It was full of grease, all the bearings were there, and there wasn't much for grind marks on the top of the cap. Wheels were straight too.
All I can think of, a c-clip was lost, and the cap was working it's way out due to rotational force. I'm gonna do more investigation tonight and this weekend, but I suspect the cap was working it's way out, and for some reason, bound up against the lower inner-C and then tore off. The cap was badly mangled at the race for the c-clip.
I've seen ujoints themselves go, or a trunnion break off the cross, or a cap explode because of rusty bearings, but never a random cap just pop off.