If its stock,then do what he said, if not...... do this.
If the rearend is stock , then the VIN is the way to go. If you are unsure of the gears, like its a different rearend, or possible gear change from previous owner, then to BE SURE, pop off the diff cover and do one of two things.
Ring gear will have manufactur casting numbers on it, when you get to the part that will have two seperate numbers, like, 10 41 . Then thats the number of pinion teeth, and then ring gear teeth. Or it could be, 41 10. Either way the larger number is the ring gear tooth count. Use that info like so...
Example, take 41 ring gear teeth and divide by 10 pinion teeth and you get 4.1, so the ratio would be 4.10
other way is to count the teeth yourself, which needless to say is cumbersome.