Curious? Did it start after you rotated the Tires? Could be the difference between the tread wear pattern on the front and the rear before they were rotated.
Does it sound mechanical? driveline maybe? Have you checked to make sure the lugnuts didn't loosen up?
These kids they hire now days either torque them way to tight or way to loose. They just can't get the "Feel" for it. They can't get from somewhere to anywhere else without a working GPS either. lol
