Easiest thing to do as you're breaking the car is take the rear diff, mark the input and output shafts with tipex. Then twist the input shaft (prop) once and measure how many revolutions the output shaft makes. If its 4.1, the the tipex lines will only be a fraction out. If its 4.4, then the tipex marks will be virtually 180 degrees apart.
That'd be the easiest way i can think of, or else I'm nearly sure you can take the backing plate off the rear diff and it will be stamped on the gear itself. But that way you'll need a new diff seal as well.
Obviously then the ratio of the diff will be the same as the gearbox..