The Mystery of the Knocking Sound is Resolved!
It was, indeed, the seatbelt latches. In fact, it was both of the 2nd Row Seat Latches and the Front Passenger Seat. The sound was hard to localize because it kept shifting its location depending on bumps on the road. As I latched each seatbelt I could eliminate the zone from where the noise might originate. This is much easier to localize if you turn the radio off.
It's not the metal latch. It's the plastic slider that holds that metal part and moves up and down the belt. There is a sewn loop on the belt where this slider rests when not in use (in its retracted position). If the belt is in a half-twist, that slider rests against the nearby plastic seat cover. If it is not twisted, there is no contact. In the half-twist position you can recreate the sound by simply bouncing that slider a bit with your fingers. A hard bump on the road causes it to do the same thing.
So the solutions might be...
a) Eliminate any twist in the belts while they are in there unused position. I don't have children, so this might be easier for me.
b) Pre-latch the belts across the seats until they are needed.
c) Add felt to the slider and maybe the metal latch.
d) Hardest way (might not be possible) -- move the slider's rest position higher on the belt by moving the loop or creating a secondary one. Not really a viable idea, methinks. Just a thought for Dodge.
But I'm glad I localized it with your help!