Keep one wheel on the ground. Lift the other (with a jack). If you can't sping the tire, either your differential is broken or you have a no slip (locker) in the rear. If you spin it 4.57/2 (or approximately 2.25) times, for a single revolution of the drive shaft then you have a 4.57 rear.
You most likely have the 4.57. It's common.
I don't know of any place that has the casting numbers linked to the gear sets.
