Bad vibes.

Flex plate only fits one way on the hole pattern in the back of the crank.... but that is correct for balancing only if the hole pattern is indexed correctly on the crank. Ditto for the crank keyway position. Production mistakes happen.... so if you don't check, you cannot be sure. Read once about a Pontiac crank where one of the throws was off almost 10 degrees from where it should have been... How it got that way, who knows, but it came from the factory like that and created a lot of problems not found until someone looked at something no so obvious....very unexpected.

It's going to be something and you have pursued the most logical things... so now for the illogical things! And just because it came out that friend's certain van does not mean it had not been touched.... sounds like it is 'open season' for anything on this one. LOL

I do hope you find it.... I never found a vibration in an old Econoline van.... I looked and looked. I just did not know about enough about these things at that time, and that was a coupla decades before the internet...

Other random thoughts: