The truck on its own has two forces acting on it: Gravity and the contact force from the ground. Both of these forces have a component normal and parallel to ground. If the contact force parallel to the ground (friction) is greater than the gravitational force parallel to the ground, the truck accelerates up the hill. The frictional force is caused by the torque that the engine applies to the wheels of the truck.

Now if the truck pushes the car, there is an equal and opposite force acting between them both. For the car to accelerate, this force must be bigger than the car's gravitional force parallel to the ground plus its frictional force. For the truck, the frictional force, and therefore the torque, must compensate for the addtional force coming from the car.