You should never replace parts until you know that is the issue. Most aoto stores will test the starters, batteries, and alternaters for free. most parts should be off the vehicle.
The starter circuit follows mostly this path:
From the battery to the fuse box, then to the key switch,
followed to either the neutral switch of an automatic or the clutch switch of a standard.
Then, depending on the year, voltage travels to a local starter relay or directly to the starter.
you need to google your truck starter circuit diagram and follow the voltage path.
test voltage before and after each part it passes through, when you find where the power stops flowing, that part is the problem.
One exception: test the power at the hot side of the starter which is hot 24/7. short directly across the 2 large gauge wires on the starter and it will turn over unless your cable that runs directly from the bat. to the starter has to be replaced.
AkitaMan @ ehow.com