FWIW, the rear locker on mine engaged almost instantly in the one low traction situation where I actually needed it, after losing traction off road, just a few days ago. I might have rolled 1 foot or so, although I actually don't remember rolling at all. And then it unlocked almost immediately when I pressed the button again to disengage (as I had to do a sharp turn just after using the locker).
I had tried the rear locker out a few times this fall and over the summer on lower traction surfaces, but I didn't really need it those times. Those times when I was just testing it out, it did require maybe a yard or two of rolling first, before it engaged. I do think the first time I tried it, it required a few yards to engage.