Vibration in Gas Pedal?

I have a 2000 Ford Explorer that just had to have a new water pump, radiator hoses, serpentine belt, and a tensioner installed. No biggie, paid the grand for it and was happy it was fixed. I started driving it home, and between the speeds of 20 and 40 mph, my gas pedal vibrates randomly. Only when im accelerating though, it stops if I let off the gas. Is there anything that the mechanic may have done, accidentally, to make this happen?