Stick a floor jack under the rear end, put it in neutral and rotate the passenger's rear wheel one complete revolution. While you do this count the number of revolutions on the drive shaft. If it goes about 3 1/2 times then you probably have about a 3.50, if it goes almost 4 times around then you have a 3.90 etc.
This method will give you a pretty good idea of what you have and is the best method I know of without actually counting the teeth on the ring and pinion.
Centerline
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