The rear driver-side door will no longer open.

Do either door handles feel "stuck" or overly stiff? compare the travel of the handles on the stuck door to the rear passenger side door. If the travel of the handle is not the same or the handle feels "stuck" the linkage is probably bound. If the handles move freely then somthing has sheared or disconnected in the linkage. Have you tryed having someone pushing in on the door from the outside and someone on the inside pulling on the handle or vise versa? Apply some good pressure to the door ( dont go caveman on it ) and try to open the door.