The bolt-backing-out theory sounds right to me. After the disaster the guibo was still in place, held by seven. They were held by nylock nuts, but the failed nut-bolt are gone so I don't know what that one was. I'll use new nylocks when I reinstall.
CD - There wasn't an escalation of vibration, noise or anything else. I don't ignore those kinds of symptoms. Came on sudden - a speed-related tapping followed by the bang, 30 seconds or so total.
PO had done a 5-speed install a while back, but beyond that I don't have much info. No driveline vibration so I believe the balance was good, but I never checked the driveshaft alignment. (Or, obviously, the condition of the guibo bolts and nuts...)
Repair options: Any ideas beyond pulling the trans and having the bit welded back on? I can use the shifter linkages to press the broken piece tight to the case, so does that make JB WEld an option (or just folly?)
Thanks for the help, brethern.