I like the above suggestions, if they don't pan out, it could be a dead spot on the starter motor commutator, as the sometimes doesn't crank scenario fits that failure mode, if the commutator stops on the dead spot.

Be danged careful doing this, make sure nothing is in front of you & you have plenty of clearance, as the vehicle could suddenly start & move/lunge. Seeing as how you have a manual tranny, when it's in one of it's no crank fits, shift into first gear & rock/move the truck so that the engine flywheel will move the starter motor a little & see if it'll suddenly crank. If so, suspect a starter commutator or brush problem.

A repair shop can remove & put the commutator on a "growler", or do a continuity test & find out if it's faulty. A visual inspection will tell if it's worn brushes.

Let us know what you find.