A hybrid electric drive system is provided for driving a medium or heavy duty vehicle. The drive system includes an internal combustion engine configured as the primary source of motive power. The engine is coupled to a front-most axle and a first drive shaft in mechanical communication with a middle drive axle. The drive system also includes an electric motor and a clutch assembly; and a second driveshaft in mechanical communication with the electric motor and a rear-most drive axle. When the electric motor is not in active operation, the clutch assembly disengages the electric motor from the rear-most drive axle.