You guys put too much importance on the loaded stance of these trucks. That's what irks me about these "leveling kits" they sell now.
It's all about the design of the truck, the pickups especially. Empty they are high in the back. If you look at the springs, they have thin arched leaves on the top, and a very thick flatter leaf on the bottom. The thin leaves give a good soft ride when empty. When you load it, the thin leaves flatten out against the thick leaf on the bottom. The truck levels out or sags a little bit, but handles the load on the thick lower leaf.
These "leveling kits" ruin that whole theory. They are fine on a street warrior, but as soon as you put a load on them the rearend will be way down in the back. And you start putting heavier springs in the back and it rides terrible empty.
So if your truck sags a little bit with some weight in it, don't worry about it, it's the way it's made to work.