Question: is the rear differential part of the powertrain

Answer: Yes, In a motor vehicle, the term powertrain or powerplant describes the main components that generate power and deliver it to the road surface, water, or air. This includes the engine, transmission, drive shafts, differentials, and the final drive (drive wheels, continuous track as in military tanks or caterpillar tractors, propeller, etc.). More recently in hybrid powertrains the battery, the electric motor and the control algorithm are also seen as elements of the powertrain.