Every truck designed or altered and equipped for and used to push, tow, carry upon, or draw vehicles by means of a crane, hoist, towbar, towline or auxiliary axle, or carried upon to render assistance to disabled vehicles, except for any truck tractor temporarily converted to a tow truck by means of a portable wrecker unit attached to the fifth wheel of the truck tractor and used only by the owner to tow a disabled vehicle also owned by him or her and never used for hire.