Hi, for a vibration felt in the seat I would have to suspect either worn motor mounts or a worn transmission mount. I would have all of the mounts inspected for this problem. Replacing the mounts will most likely cure the vibration. It's also possible that there is a vibration coming from the motor but it would not be as common. The serpentine belt could be removed for a quick test to see if any of the rotating components are causing a vibration. The mounts are the most likely problem however.
Let me know if you have any questions.