The tires are rated at a specific load capacity at their maximum air pressure. Those will be rated at something like 3400lbs at 80psi. As you lower the air pressure the overall capacity will be reduced. The factory recommendation is 45psi front and 55psi rear. Here is an example from Firestone on what I'm referring to. Scroll down to the Light Truck section and look for your size:

http://www.firestonetrucktires.com/p...Load_Table.pdf