A hybrid drive system for a vehicle having a front drive axle, a middle drive axle and a rear drive axle, the hybrid drive system comprising: an internal combustion engine; a first drive shaft in mechanical communication with the engine and the middle drive axle; an electric motor including a clutch assembly; a second driveshaft in mechanical communication with the electric motor and the rear drive axle, the rear drive axle and the middle drive axle being mechanically isolated from one another; an energy storage unit for storing and providing energizing power to the electric motor, the energy storage unit being in electrical communication with the electric motor; and characterized by a drive controller for controlling the electric motor and the clutch assembly, the drive controller controlling engagement of the clutch assembly only: when supplemental motive power is demanded to supplement primary motive power generated by the internal combustion engine to transfer power from the electric motor to the rear drive axle, and when the vehicle is decelerating to transfer motive power from the rear drive axle to the electric motor to recharge the energy storage unit, and the drive controller is configured to control disengagement of the clutch assembly at other times including when the internal combustion engine satisfies a demanded motive power during cruising or acceleration of the vehicle in order to isolate the electric motor from the rear drive axle. an internal combustion engine; a first drive shaft in mechanical communication with the engine and the middle drive axle; an electric motor including a clutch assembly; a second driveshaft in mechanical communication with the electric motor and the rear drive axle, the rear drive axle and the middle drive axle being mechanically isolated from one another; an energy storage unit for storing and providing energizing power to the electric motor, the energy storage unit being in electrical communication with the electric motor; and characterized by a drive controller for controlling the electric motor and the clutch assembly, the drive controller controlling engagement of the clutch assembly only: when supplemental motive power is demanded to supplement primary motive power generated by the internal combustion engine to transfer power from the electric motor to the rear drive axle, and when the vehicle is decelerating to transfer motive power from the rear drive axle to the electric motor to recharge the energy storage unit, and the drive controller is configured to control disengagement of the clutch assembly at other times including when the internal combustion engine satisfies a demanded motive power during cruising or acceleration of the vehicle in order to isolate the electric motor from the rear drive axle. when supplemental motive power is demanded to supplement primary motive power generated by the internal combustion engine to transfer power from the electric motor to the rear drive axle, and when the vehicle is decelerating to transfer motive power from the rear drive axle to the electric motor to recharge the energy storage unit, and the drive controller is configured to control disengagement of the clutch assembly at other times including when the internal combustion engine satisfies a demanded motive power during cruising or acceleration of the vehicle in order to isolate the electric motor from the rear drive axle.