I would guess the bed is out of alignment which is why you're seeing the difference. As for the shake, probably the tires. Balance, out of round, not mounted right on the rim. I would also measure from the frame to the outside of the tire on both sides. See if the distance is the same within...
Does anyone have the wire information from the amp to the speakers? I'm assuming the speakers are all powered from that little Sony amp on the back of the subwoofer box? I have a 2014 truck with the Sony and Nav setup.
Thanks,