Interesting topic and I don't mean to misdirect it but, I have what I've referred to as a death wobble while towing my HHR behind the motorhome. It happens so infrequenly I hesitate to mention it but when it does occurr, it shakes the motor home too.
It feels as if the towed vehicle has a flat, when it happens, I can see it rocking back and forth in the rear camera! It's very unsettling but goes away as quickly as it happens. My best guess is that the two bar is binding and somehow causing the HHR to "hop". I can't say for sure, but if I spray the tow bar and assorted joints with silicon lube before each tow, no issues!
It's fresh on my mind only because it happened on the way home from Arizona a couple weeks ago....the few miles before getting home after a 400 mile jaunt. The tow bar is a Demco X-caliber.
Didn't mean to hijack this thread????