Really, the ICON Stage 5 Kit isn't going to help much with rear-end sag when loaded down. As awesome as this kit, it's really meant for dedicated off-road use on trucks that really get flogged around the trails, and won't do anything to help your Super Duty's towing capabilities (though it won't hurt it either). While there's not much you can do to actually increase your truck's GVWR, doing either an Add-a-Leaf Kit or set of Air Springs (or even both) would likely be a much more effective use of your time and money. As far as front leveling kits go, there's plenty of options out there, though your final choice is going to depend on what kind of balance of off-road performance and on-road ride quality you're looking for.