My 2005 Nissan Pathfinder vibrates whenever I reach 40 MPH and above. It seems to be coming from the rear, but I can feel the vibration in the driver's seat and steering wheel. Also, the vibration ONLY occurs when I press down on the gas pedal and keep it at a steady speed. Once the vibration starts, it will stop immediately after I let go of the gas pedal. The vibration also stops if I shift into neutral.
Several things could be the cause. I would suggest having a tire balancing done first. It could be that simple. You could also have a bad u-joint in your driveshaft or a bad bearing in your differential. IF tire balancing doesn't fix it then check the rear differential gear oil. If it shows a lot of metal particles then the pinion bearing may be going bad.