I think you are making this far more complicated than it really is. The fact is, your tires should be your first concern. My bet is that if you look at the door-sticker weight numbers, and compare them to the load ratings on the side of the tire, they will be the same. My point is you need to weigh the truck, at least the rear axle to see if you are within the capacity of the tires. If you load is greater than your tire capacity, then you need tires with a heavier load capacity.
If you modify the overload springs to make contact sooner, it will only reduce the amount the rear of the truck squats when loaded. The same amount of weight is on the tires.
Personally, I suggest you look at your tires. Lots of potential problems if they are overloaded, including excessive sway.
Wayne
