Those of you who thought my problem was the universal joint were correct.
It turns out the transmission shop that rebuilt my transmission and universal joint assumed the mechanic who would be reinstalling my engine and transmission would be taking the universal joint apart in the process. So they didn't tighten the bolts or bend the lock tabs. Unfortunately, he somehow managed to put the engine and transmission back in without doing that. The transmission guy was impressed, but thought the mechanic should have noticed the lock tabs weren't bent over.
Of course, the transmission shop thinks they did nothing wrong, and the mechanic thinks he did nothing wrong. But the mechanic agreed not to charge me for the labor if I pay for the parts (it bunged up the universal joint end pretty bad, and the bolts don't seem to have any threads on them anymore).
Thanks for all the feedback.
