I have a vibration, I believe it is in the rear end. It starts around 40mph and gets worse as you go faster. Sometime it will find a spot and not do it at all, but then it will start again. I can feel the seat shaking and I can also see the passenger seat shaking. If I have a bottle of water in the cup holder, it shakes. I just had different tires put on and it still vibrates. Took it to auto tire (not the place I got my tires), and had them rebalanced and rotated. They said tires, brakes, rotors, all look good...Still vibrating. Where should I look next? I checked for open recalls because I read where the rear axle pinion nut, but my truck doesn't show up as any. It has 110000 miles on it. Any advice is appreciated.