I figured it out. There is a bolt holding the subwoofer to the actual enclosure. I removed the three screws securing the enclosure to the vehicle. When I flipped the enclosure over, there it was.. the bolt that had the bain of my existence. As soon as I removed the rear bolt, the sub popped right out.
I filled the enclosure with poly fil and mounted the new Kicker sub. It wasn't an immediate improvement, as I am a firm believer in the speaker break in theory, but it does go lower than the factory sub. A new amp with a level control is next.