Ah, er, bradfordo, nothing personal, but if you cannot find the rear differential, ah maybe you should just leave it alone....
But I believe your manual will show you that the rear diff. is located between the rear wheels (at the back of the car).
And, ah, you will need an allen socket, a sort of big one, you know, maybe a 14 mm.
Also, depending on the shape of your diff. oil container, you may also need a suction gun, to reach up over the exhaust, it can be ackward on the ground.
And on the transmission, located behind the motor, be sure to also drain the torque converter. To do this you need to rotate the engine (clockwise, ah , to the right) until the drain plug is visible out of the little hole at the bottom.