A quick web search didn't find a formal definition, only information pages for specific parks with descriptions. I agree it's likely park rangers would use common sense in practice such that a vehicle with an extremely low first couple of gears, lockable differentials, appropriate clearance, and suitable tires wouldn't get cited as "not 4WD" because it doesn't have a selectable 2WD mode.

Incidentally, I once owned a Subaru from the 1980s which had a lockable center differential and separate high/low gearshift, which was synchronized and could be shifted in motion. It was not designed for serious off-road use, illustrating the folly of relying on criteria like these.