Once the brakes start to drag, stop the truck, chock a wheel front and rear, and loosen the bleeder screws on the calipers. It's possible you're trapping fluid in one or both calipers due to a flex line collapsing inside. If you get a spurt of fluid and the brake drag releases, replace the flex line on the offending side, or both of them if that hasn't been done recently. I've seen several examples of that problem over several years, one of which got so bad a transmission failed because the car owner was too stubborn to stop and call for a tow truck, and kept on driving with the brakes dragging.
Jerry
