When you ditch the coilover shocks, get a good Monotube shock. Bilstein, KYB, Fox etc. Those will for sure make the ride softer, while providing better loaded control at the same time. But "shocks" (which are really dampers, and don't actually absorb shock....) won't help with the harshness that gets transmitted through leaf springs, and that's where the Sulastics come it. The act like an isolator for higher frequency vibrations, which we feel as harshness, or banging, when you hit an expansion joints cracks etc. And they also help reduce wheel hop on washboards (but the nature of a 800 pounds of unspring weight being a dually, it'll still happen, just not as easy).
Doing good monotube shocks in conjunction with Sulastics, your dually will ride at least as well as a stock 3/4-ton, if not better.
