Wow it looks like you've been getting some good advice and things to check. Was just talking with my Dad last night and he was saying some of the things he'd look for . . .
bad sparkplug - cracked or internally defective
bad sparkplug wire - check brass crimps
hairline crack on distributor cap - look for carbon trace(s)
small crack on a fan blade
bad fan clutch
remove each accessory belt - test for elimination of problem
alignment dowel mishapen/missing between engine/trans
pilot/throwout bearing worn/mishapen
mismatch of balancer + flywheel
slight damper/balancer wobble
mismatched incorrect flywheel bolts
clutch/pressure plate bolt loose/broken
bad/intermittent Duraspark module
worn lower bearing(s) loose rods etc
Here's a nice worthy read titled "Bad Vibrations" (pun on Beach Boys "Good Vibrations" I guess lol)
"Bad Vibrations"
https://www.fordmuscle.com/archives/...ions/index.php
A final note is more than a few posters online have mentioned using a vibration app on their phones. Might be a clever idea (or not!) I don't know since I haven't tried it. For instance here was what one guy was saying . . .
Hope you or someone in the future might find something here helpful. If not, I do wish you luck finding it!
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