Posting here for future knowledge:
the issue was in fact the harmonic balancer. My Handiman said the old one looked like it had been “beat on with a hammer.”
replacing the balancer did fix the issue.
I guess the moral of the story, is if you have a weird engine vibration you can't track down, it might be the balancer. Even if you don’t have the usual sound that occurs with a bad balancer.
https://a.co/d/it4bmKn
That's the kit I used. It's easy to assemble yourself, I just used hot glue to hold the LED's in place. I mounted one above the interior, one above each fog light pointing down, and one in each taillight, pointing outward.