If you are going to keep your truck for a while, having a new/rebuilt starter is a good idea. If it does not solve your problem, you have a good spare.
Otherwise, even though you have "checked" most of the following items, carefully adhering to these suggestions will eliminate other items:
Note: when the engines and other starting parts are warm, the engine often have more "starting resistance" - at the margin, starting when warm will become difficult/impossible.
1. Good cables from the battery - BOTH the positive lead and the negative lead (1 gauge to 00 gauge, copper). Smaller cables will cause starting failure in a marginal system.
2. And, clean/shiny contacts for the cable-connections - clean metal, protected with conductive, anti-oxidant grease ( Kopr-Kote, Kopr-Shield (no longer listed by Eastwood), or NAPA: BK 7652569). Check these out, clean-them until all contact surfaces are shiny, then put on the conductive grease.
Next to consider (although, I now do this first on my trucks):
3. From the battery: run the positive cable directly to the starter (the normal arrangement) AND a negative cable to a starter-to-clutch-housing bolt (not the normal arrangement). Be sure to still use other frame-to-block ground cables (and maybe a head-to-firewall ground cable).
NOTE: unless you have 6v jumper cables, normal/modern jumper cables are inadequate.
Then, rebuild the starter (or have it rebuilt); or, buy a new/rebuilt starter.
