Interestingly, I had my son sit in the back seat while I drove around a bit last night. While he also heard the click from the rear driver side door, he said there was a fainter click from the passenger side as well... Not sure whether that means that both doors are trying to lock, or something else. Either way, am I safe assuming that the side with the louder "click" is the problem side? (For parts ordering, if the dealership won't cover it under warranty since I'm at 42k miles.)
Another small note: I had my son try opening the rear doors while driving, but after passing 10mph, to see whether the doors actually were locked or not. He discovered that both doors were locked on the first try, but that he could open either door by trying the handle a second time. Apparently, attempting to open the door while in motion doesn't work if you pull the handle once, but will work if you pull a second time. Is that some sort of safety feature?