Changing rear ends does not give you more gears. It just changes what gear you'll be in (on the highway). I.e. It just changes which gear would have you in the powerband at any given speed. Going from a 3.21 to a 3.92 is not going to help you on the highway. If the powerband is 5K, then changing from a 3.21 to a 3.92 just means you'll be in, say, 5th gear instead of 4th, to be in the powerband.
And, if you're going down the road at 60MPH, and the engine is right in its powerband, it doesn't matter one bit whether you're in 5th gear with a 3.92, or 4th gear with a 3.21. It's the overall gear ratio that matters.
Changing from a 3.21 to a 3.92 would only be helpful to you for taking off from a dead stop, when towing something heavy. That would let you get into the powerband at a lower speed (i.e. sooner after takeoff).