Replacing the brake hoses would be a good starting point, do both sides. Flush your brake fluid in the entire system while you are at it, the fluid is likely contaminated and isn't helping.

In the real life on the truck, the only thing that pulls the caliper piston back to release the brake is the square "O" ring on the piston cup. When you step on the brake pedal, the fluid pushes the piston cup towards the rotor and distorts the square "O" ring, Then when you let off the pedal, the pressure is relieved (if the brake hose lets the fluid flow like it should), and the square "O" ring returns to its normal shape and pulls the piston cup back away from the rotor.

Lack of regular use allows anything floating around in the brake fluid (moisture, breaking down rubber), to settle in around the "O" ring, which may not allow it to deform and pull the piston cup back away from the rotor like it was designed to do. It probably starts out as a slight drag the first few times you drive the truck after its sat a while, but then appears to function correctly. The heat build up from the slightly dragging brake probably gets the fluid hot enough to clear the moisture and remix the other debris into the fluid while the truck is in use. That "correct function" is probably short lived, especially if the truck is just used a few times before it sits again. Eventually the debris builds up enough the "O" ring can't function properly. Its the nature of the operation.

Flushing the brake fluid every 4 or 5 years may solve your issue, because it rids the brake system of debris that may be floating around in the fluid. I believe there is probably a miles AND a time frame recommendation of the required brake fluid flush in your owners manual (that very few people me included read). You know, all those things the car companies want us to spend our money on (coolant flush, trans fluid flush, brake fluid flush, belt & hose replacement time frames) that don't make much sense to us? I know the fluid flush requirement, was at least printed in the FSM (or used to be).