The vacuum hub engagement looks like the weak link in the system to me. If I get stuck somewhere, unexpectedly, I'll simply engage 4WD inside the cab, and drive out. But if I'm going off road intentionally, I always engage the hubs early. This lets the system get all lubed up and it positively locks in the front wheels and keeps them locked in. Neither the manual or the automatic system engages until the wheels rotate enough to line up the splines in the hubs, but that occurs almost immediately and should never be noticed. If we are stuck in 2WD, and engage 4WD, the axle shafts will turn in the front axle, and line up the splines in the hubs immediately. When we turn the knob on the hub to "lock" we are not really engaging the hub. We are compressing a spring that is pushing on a splined collar. As soon as the axle rotates a few degrees, relative to the wheel, the splines line up and the collar slides in, locking the wheel to the axle shaft. The vacuum system can slide that collar too, if it is working correctly.
It seems unclear to me if 4WD high can be engaged while driving with the hubs in "Auto". In theory, it seems like it would be OK, as long as the system can engage the transfer case while driving, with synchros, and then do the vacuum hubs. But the manual seems vague on this. If the hubs are engaged, I see no reason why we cannot engage 4 high at highway speed. This is handy for mountain driving where slippery areas come and go. Of course, 4 low is a different story.