They call it a door wedge I believe. As you said it stiffens up the door to door post. There is another piece that goes on the door itself that is available in the aftermarket. I do not believe the one on the pillar is available anywhere but used.
Thank you for the replies. I cannot find the component in the manual or part explosion diagrams. When I search for door wedges I usually find the small hard rubber wedges. As a note, this is on the door, not the pillar.
