You really need to pick up the factory service manual.
There's a proportioning valve connected to one of the rear springs; when there's more weight, it allows more brake fluid pressure through the lines to the rear brakes, to keep it from locking up when empty vis not braking at all when fully loaded.
As to the weight; check the door sticker for your GVWR and see how that compares to the spring weight.
Another suggestion: While replacing, install new bushings in the rear shackles. I'm fond of the Energy Suspension or Prothane bushings myself, to help firm up the rear end; but it's your truck, your choice as to rubber, poly, or possibly finding delrin or solid bushings.