http://opengameart.org/content/monster-truck-1 you will have to do any animations yourself.

I have one in my portfolio.

Turns out there is a third one at http://opengameart.org/content/monster-truck which I found using a google search... So three CC-0 Monster trucks to choose between, none of which show up on the search.

Also one in http://opengameart.org/content/hill-climb-racing-car-sprites depending upon if you want a guy sticking out of it and to play with the parts.