What is the weight on the tire? Single or dual rear wheels?
If you know the axle weights, you can look up in a table from the tire manufacturer (or really any maker of the same size and type and weight rating of tire) and see what the appropriate inflation pressure is for your weight. 80 psi would only be appropriate around the maximum rated weight capacity of the tire, which I'm pretty sure you're nowhere near in everyday running around.
If you don't know the axle weights, the door sticker would give reasonable information for the maximum payload of the truck, if these are the stock sized tires for the truck.