It shall be unlawful for any person to drive a passenger motor vehicle on the highways of this State which has been elevated or lowered either in front or back more than six inches by a modification, alteration or change in the physical structure of the vehicle. Any person violating the provisions of this section shall be deemed guilty of a misdemeanor and upon conviction shall be fined not less than twenty-five dollars nor more than fifty dollars. Provided, however, this shall not apply to motor vehicles commonly called "pickup trucks".