Starter magnet alignment?
 

My batteries seem very charged, and the solenoid "clunks" nice and clearly.

I think the starter magnets are at that perfect storm spot where they don't touch, and you have to rock the truck (it's a manual trans) to nudge the magnets back into alignment.

I had this happen frequently on my old Ford van (1966) in high school, but I would have though this problem had been eliminated.

Comments or suggestions? My 1999 F-350 SuperDuty Crew Cab is too big to rock (that much) by myself.