It is definitely locked, if you turn on pavement it won't slip. I don't know about durability, but it unlocks over a certain speed so I assume that helps.
On my last FX4 the switch went out and left it on all the time for a bit. I can assure you it locks that bitch and chirps the tires all the way around and made it hard to turn. Cheap switch to replace though. Also they turn off at like 35 mph automatically.
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