OP, you've opened up one of the biggest debates that goes on here. The question of can a machine be modified to perform better than when it left the factory or is it impossible to change a machine's design.
Your truck can certainly be made stronger. You can put on tires with more weight capacity. You can change the wheels to ones with more weight capacity. You can add air bags or helper springs or replace the springs entirely for more capacity.
You could even swap out the entire rear axle assembly which would get you a stronger housing, axles, diff., brakes, etc.
All of this WILL increase the ability of the truck to safely carry a load. However, it won't change the sticker on the door which some people put more faith in than actual physics.
However, if you're truck isn't up to the intended task, the simplest, easiest, most cost effective, and probably safest thing to do is to trade it for a heavier duty truck.