Subaru Repair: Gas pedal vibrating, seat of your pants, tire rotation

Question
I got a tire rotation and an alignment done... immediately afterward my gas
pedal starting vibrating. Whenever I speed up the vibrations increase. When I
remove my foot from the gas pedal the vibrations stop. Did they possibly mess
up my alignment? Is this all because its just extremely cold? (I live in Canada)
Any ideas would be helpful, thanks.

Answer
I would start with making sure the wheels are on tight. The only place you feel a vibration is in the gas pedal? Not in the steering wheel or in the seat of your pants? Is it possible the vibration is worse when accelerating(foot on gas pedal)?

Justin