Well, I got my answer today, and fortunately for me, it wasn't the $8000+ answer!
Turns out it was one very badly blown out injector on the #2 cylinder (the tech said it was blowing bubbles at idle), so the tech
swapped them all out for new stock units, and the problem was eliminated.
The Master tech at the shop said it would have been extremely unusual for such a young motor (78K) to suffer from a mechanical failure (especially with this model motor), and was also surprised that the injector had blown out.
A check of the injector pump and injector pump timing showed them both to be OK, and the timing chain slack was still only 2 degrees.
Total damage was $385 for the injectors, plus new fuel filters, plus labor.
Guess I'll just have to wonder what ate my injector(s)...