Look up the load inflation tables for the tire and go from there. They pressures are based on weight and the weight listed is per tire usually not per axle so if the rear axle of your truck weighs 3500lbs that is 1750 per tire and that is weight you would use for the pressure rating
example- in link scroll down to the light truck tire section starting on page L7 and from there find your tire size it will show you the inflation pressures needed for various weights up to the tire size max psi
http://fifthwheelst.com/documents/Goodyear_Tire_Inflation___Load_Charts.pdf