If you dont feel it in the steering wheel and you feel it in the seat how is the carrier bearing? when you did flex disk did you replace it? If this is new I would look at the new flex disk or like everyone said the tires again. Most likely the rears, easiest way to lose a weight, or the rear wheel bearings to the rears got loose or are going bad. Alignment would cause the wheel to "wobble" in the front for lack of a technical term.