Honestly I just drive my V8 4WD a hundred to a few hundred feet or so in a straight line (important to keep it straight on pavement) at least once a month with the center differential locked to exercise it. While its an oft not used function, there is a mechanical linkage to lock-up the planetary gearset that makes up the central differential.
As for V6 4WD vehicles, it wouldn't hurt to do the same in 4Lo to exercise the ADD and Transfercase. We used to see quite a few 4WDs that were "fixed" by just tapping on the transfer case and then exercising the actuator a few times (easy diagnostic work
)
