BigB,
Hope you kept the receipt for that AutoZone starter - you'll be replacing it again soon enough. In my experience, anything reman from that chain isn't worth much except as a doorstop. Also as CCatlett let on, AutoZone can't test a 24V starter properly, their machine isn't capable of it.
Have you converted to 12V from the military 24V system? If you use a 12V starter in a 24V system, you'll burn up starters on a regular basis until you get a true 24V one. AutoZone doesn't have it in their system (at least they didn't a few months ago when I needed one), but NAPA does under the 'heavy truck' catalog.
The big hint that you've got a 12V starter in a 24V truck is that it spins up real fast - in fact, about twice as fast as it should. This works fine for a short period of time, if everything is within spec - but just a bit of wear here, a touch too much clearance there, and the bendix won't engage, or you'll crank too long and something will overheat.