It is not likely to be the brushes. Inspect before you buy. The fsm has a length spec. Note two large screw heads flush with the motor surface. Remove with a large flat head and the brush and spring will be underneath. To test the brushes, use a set of jumper cables and the battery. Hook one cable to starter motor frame and battery -. Hook one end of the other cable to batt +, and just touch the other end to the copper stud on the starter that the truck battery + terminal was NOT connected to, the one closest to the starter motor connected to the wire that enters the motor. Starter motor should run vigorously. Take care, the motor will jump as it starts to run.

It is most likely the solenoid on the starter. There are two large copper studs on the starter, one connected to the battery via the fat cable, and one with a conductor to the motor. Internal to the solenoid, there's a moving metal core with a copper disk on the front end. When you juice the solenoid wire, the core is pulled forward by the magnetic coil in the solenoid. This does two things. One, it extends the starter gear and engages the teeth on the ring gear on the flywheel. Once this is done, as the core hits the forward end of it's travel, the copper disk comes into contact with the inner end of the two copper studs, completing the current path from battery to starter motor, and the motor spins and turns the engine over to crank it. It does it in this order so the gear is engaged before the motor spins. Due to the heavy current carried to the starter, you get arcs every time the copper disc comes into contact with the inner ends of the studs. Even thought the copper disk is made to rotate and use a newer surface for contact, over time and multiple starting events, the arcing scars build up and keep the current from flowing. Thus, you hear the starter click, but the motor does not turn.

Two fixes. One, take the solenoid apart and sand the ends of the studs and the copper disk clean and shiny. Two, buy a new solenoid, if available. One is cheaper, two is easier.