The battle is over, and I am victorious.
I finally had a few hours cleared up to devote to this thing and I gave it a few more hard bangs with still no effect. I proceeded to take off the rear passenger's door in hopes that I could get a better view of what exactly was stuck in there, and indeed, you are able to see just enough metal that my suspicions were partially confirmed, I was pretty sure that the latch was jammed over the striker. I grabbed a long skinny flathead screwdriver and found some real estate and started hammering. Stuck solid, but I just kept hammering half expecting something to break or break loose. Finally, and after many curses, that son of a gun broke free, and the door just swung open like there was never a problem at all.
So, here's the postmortem:
In the latch area, there appears to be some broken white plastic. On the driver's side there is also a white plastic cover, or plug, or rectangular doohicky that seems to cover a small spring. I have no idea what this spring does exactly, but it is possible that A.) This doohicky is what broke and the small pieces somehow jammed the latch up, or B.) I broke this doohicky while hammering the latch with the screwdriver. Option C.) is that the latch just jammed up on its own accord after being opened and slammed the first few times in about 15 years.
The most bizarre part of the whole affair is that now the door works totally fine, it opens, latches, and locks from both the interior and exterior and the only change I made was pull some plastic chunks out and squirted a bunch of grease in there.
Thank you guys for all the support and suggestions.