My 2013 Honda Accord Sport, 6-speed manual, 120K miles, has started vibrating only when accelerating at higher road speeds (70 mph and above). As an ex-tech, I was pretty sure it was rear tire balance, because there is no vibration in the steering wheel or that seems to come from the front. But then on a trip this week I noticed that it only "bounces" or vibrates through the body when I am accelerating or under greater load, such as going up a hill at those speeds. Any clues as to the most likely cause would be greatly appreciated. Thanks.