I FIXED MY SHAKING PASSENGER SEAT!!!!!
After doing the rear seat delete, I mistakenly removed the towel I had wedged and the seat started shaking again. I about lost my $hit on a recent 3 hour drive.
So after messing with the seat, I believe I found the problem and have a fix - I am two days shake free! It appears to be a manufacturing/design issue. After sliding the seat to where you want it, grab one side of bottom and push it back and then forward, repeat on the other side if nothing happens. What should happen is a 'click' of one seat guide in the rail being set into position. Each seat rail has places for the seat to 'click' into place, however, because of design or manufacturing issues the detents where the seat sits in each rail don't seem to line up. So one side will be 'set', but the other side won't. This happens every-time the seat is moved forward or backwards, so you will have to repeat the process if you move your seat.
I'm curious if this will work for others.
Now the bigger question, should this be reported to NHSTA? I'm not an automotive engineer, but I am going to guess that in the event of an accident, you would want your seat fully locked in place, not just half-way, right?