I loosened a back seat mounting bolt & easily pulled the mystery part out from under one of the seat mounting brackets.
Here's what the part looks like;
It's called an Insulatio-Side Panel, Mopar Part No 5519422A.
The next question is where does this panel belong? I'm hoping it was replaced by a speaker (or something), but it'll be nice to know if the part is needed somewhere else on the Jeep.