The dealer or a shop manual might tell you the gear ratio from the serial number. Some differentials have the ratio on a stamped metal tag on one of the differential bolts. Since it is a 4WD if you change the differential gear ratio in the rear you might have to change them in the front too. Probably not worth it. The number of teeth on the ring gear divided by the number of teeth on the pinion gear = the gear ratio. You can approximate the gear ratio by turning the drive shaft exactly one turn and counting the number of turns and fractions there of that the wheel turns.