No vibration at idle. None at acceleration in any gear below 4000rpm. Seems to be very evident at higher road speeds, under max acceleration, and was *very* pronounced with the trailer. Having the tires rebalanced tomorrow (and rotated), but, i'm 99.98% sure this isn't a tire or wheel problem. This is something moving in the drivetrain. Feels to me like either a bad mount, or a bad support bearing for a driveshaft (had that on the old volvo, and the sensation is similar to that). It's definitely related to the amount of torque being put on something under stress of acceleration. And it's not in off the line torque, as that would be evident in first gear, and second gear, and it's not noticable in first or second gear. 3rd, and 4th, (and probably 5th and 6th - if i wanted to get a ticket).