Or you can do it the fun way--Jack up the back of the car (both wheels). Put the car in neutral and have someone turn the tire/wheel exactly one complete revolution while you lie under the safely supported car and count how many times the drive shaft turns completely around (put a chalk mark on it as a reference). If it turns 3 and a third times, you have the 3.36 gears. 3 and a half times and you have 3.55, and so on. If both wheels turn in the same direction, you know you have Safe-T-Trac (posi). If they turn in opposite directions, you have an open rear end. 3500 rpm at 70 sounds like you might have a 3.90 gears (just short of 4 complete drive shaft revolutions).
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