Update
They just got to it even tho they've had it since Tuesday. They were unable to recreate the problem so I had to go in and do it for them. But the tech riding with me was immediately pretty sure he knew what the problem was once I recreated it...good news: not the transmission.
It sounds to him like there's a failure in the 4wd vacuum system. Apparently the hubs default to 4wd as that's seen as more useful (if the system fails they want it to default to 4wd in case you're stuck somewhere). So in 2wd the vehicles run a vacuum to hold 2wd. It sounds to him like the system is leaking somewhere so it's losing the vacuum and the hub on the front driver's side wheel is trying to engage. That's the scraping/grinding noise I'm hearing, the hub attempting to engage while moving. The system isn't showing 4wd because the transfer case is still in 2wd, it's just that individual hub trying to engage because the vacuum that holds it in 2wd is failing/failed.
So, they have to tear into the 4wd system and see where the vacuum is bleeding off, fix that, and I should be ship-shape.
Annoying, but reassuring it's not something more serious like the transmission.
I know there are some seriously mechanically-minded folks on here...if that scenario sounds reasonable, please let me know. If that sounds like hogwash, that would also be helpful to know.