Well I believe I figured it out. I was smacking the trailing arm with my palm, seeing if I could get that to rattle, but I heard a rattle toward the bumper. Eventually I found that one of the bumperettes could be wiggled up and down. The bumperettes bolt to a reinforcement bar behind the bumper. I took the bumper off and, sure enough, the two bolts that attach the reinforcement to the frame were loose.
I have no idea how they got loose loose. Kind of disturbing, actually. Maybe it was a shoddy repair—during disassembly of the spoiler to get the bumper off, I noticed two of the three rivets that hold the spoiler trim had their centers knocked out already.
So it looks like I didn't need to do the trailing arms, but seeing how common an issue it is, it may have been inevitable.
I hope this helps someone!
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