Well this thread can be closed out, after all this time I've finally found the source of this insanity-inducing noise. The driveshaft adjusting nut was loose. This is a large nut that secures the splined connection between the front half and rear half of the two-piece driveshaft. It can slowly loosen over time and allow the male splined end of the rear/intermediate shaft to rattle inside the split female splined end of the front shaft. The nut can be loose but appear to be fine even if you can't turn it by hand. Good news is that it's a quick cheap fix. Bad news is IT'S THE MOST DIFFICULT DAMN PROBLEM ON A VEHICLE THAT I'VE EVER HAD TO DIAGNOSE! I need a drink.
I am taking my car into the mechanic today. If that fixes the issue, I will buy you a bar! Thanks for the update!!