If both axles are "live" and are connected via drive shafts, not chains, then both rears need to have the same gear ratios. If your truck has the "standard" SM420 4-speed transmission it should be upgraded to a HD Spicer. The SM420 isn't strong enough to handle the torque and loads that the added rear allows the truck to carry. I don't believe Chevrolet sold a factory built tandem with a 6-cylinder either...only V-8's, again for the higher torque rating.
Mike B
