Yikes!
I think the ujoints were starting to go and the bolts vibrated out. Still, even the driveshaft bolts came out easily, way too easily. I think when I'm done I'm going to give everything down there a onceover. I've got a big container of Loctite blue.
Another thing, it's a Chevy so it's got the plastic lock on the ujoints, you have to burn them out. I did read that occasionally they POP! when heated, but when it did it the first time I near jumped out of my skin
Pete