I'd whack on the starter with some kind of tool (small hammer, tire tool). It normally takes awhile of malfunctioning (starter sitting in bad spot) before the whole starter is toast. And agree the bench test may show nothing (mine- f150 did not). Smack on it while having someone crank the key- easier than doing it single-handed. If you haven't had any previous starter trouble- and the starter is the problem- it should reengage and start. If it does, you need to get a new starter soon, or get used to "whackin' it". If it doesn't start- it's likely not the starter.
I'm voting for bad connection somewhere, or low battery voltage.