NPS defines 4WD specifically as part time four wheel drive with a transfer case and low range, FWIW. Low range is a big part of it. And yes, their definition technically excludes higher end "full time 4WD" systems in some cases, though I suspect everyone would look the other way at those.
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.
> A Jeep, sport utility vehicle (SUV), or truck type with at least 15-inch tire rims and at least eight inches of clearance from the lowest point of the frame, body, suspension, or differential to the ground. Four wheel drive vehicles have a driveshaft that can directly power each wheel at the same time and a transfer case that can shift between powering two wheel or four wheels in low or high gear. All wheel drive (AWD) vehicles do not meet this definition.
I completely agree they'll use their discretion, but either way, that definition is specifically a part time 4WD system with low range.
