If it's the brake, you should be able to smell burning pads immediately when you get out of the truck and sniff by the wheel.
Second cause of a constant pull, especially if there's no burning smell, no pulsing or changes in noise during braking, or no apparent issues upon inspecting the caliper, would be a broken shock. Try to drive with the driver's window open on a side street or empty parking lot that might have a bump or two and listen for clunking that would indicate a shock or suspension issue. A broken shock would also show signs of hydraulic fluid leakage.