I have seen this before on Harley's. It normally requires rebuilding the brake caliper. It is caused by the piston pushing at an odd angle. I do not know if you can get a rebuild kit for the truck calipers but for Harley's the kit contains an oring and new piston that must be installed to replace the old one. If they do offer a rebuild kit make sure to clean out the caliper where the piston goes in with emery cloth and brake cleaner. A warped rotor will cause pulsating or shaking vibration under braking.