It is not when tires get airborne that you need lockers. It is a common misconception and LandRover harped on that one for years by saying "with longer wheel travel your tires stay on the ground longer and you don't lose traction". Fact is that tires will start spinning when the amount of torque generated with your gas foot is higher than the amount of traction your tires have.
For example (I'll keep it as simple as possible even if the engineers don't like it):
You drive with all 4 wheels on level ground. A 10" high rock catches your eye. You have to drive over it. We all understand. You plan on driving over it with your left wheels. Before you hit the rock and all tires have equal high traction, all can absorb an equal high amount of torque.
Location front axle:
Left wheel gets to the top of the rock - spring is compressed (carrying more than 1/2 of the vehicle's weight)
Visual: tire sits already inside the fender well
Result: more traction than when axle was level (weight plus friction between rubber and ground = traction)
Right wheel rolls on flat ground - spring is extended (carrying less than 1/2 of the vehicle's weight)
Visual: tire moved way down from original position
Result: less traction than when axle was level
Now it gets tricky.
A rule first: The unlocked differential always distributes torque equally left and right. It allows for unequal velocity - but torque distribution is always equal.
The right wheel in its new situation can not deal with the same amount of torque as before. Depending on how hard you are on the gas it has an infinite range of reaction: From slipping slightly and rotating barely noticeable faster than the left to spinning twice as fast as before and the left stops moving completely.
The locked diff would have guaranteed that both wheels rotate at the same speed. Well, you forgot to hit the locker.
If the right slips a little but the vehicle keeps moving you are off the hook. Just remember next time to lock before you climb.
In case you are so hard on the gas that the right spins and the vehicle becomes immobile, you have a couple of choices choices:
A. Reduce the gas so far as to eliminate the wheel spin and most likely the car will keep moving.
B. Get off the gas and on the brake, engage locker and proceed driving carefully.
By the way, the rear two tires are of no help in this situation, because in the rear the diagonally opposed tire (left rear) will also slip and spin. Always! No exceptions.
More about choice A. :
Since the open differential always distributes torque equally, the left wheel will get exactly as much torque as you can generate on the right up to the threshhold before it starts spinning. If the amount of torque left and right together (plus rear left and right together) is enough to move the car - it will move.
For example, on an uphill drive, more torque is needed to move the car than on flat ground (Duh!), and if you have to drive up the same height rock while going uphill, the amount of torque generated right before the threshhold of spinning is most likely not enough to move the car.
That's why you can get through rough stuff on level ground almost with a Honda CRV - but need lockers on both axles for steep climbs.
Locking your diff –
On dry pavement - Never! You’ll lose all ability to steer. Bad!
On pavement with snow and ice engage your locker(s) whenever you think there is a need to keep the tires from spinning as long as you are driving straight. Disengage the front way before you would get into a turn, or severe understeer would occur.
Lockers can be engaged while the vehicle is moving - at any speed. But like in difficult off-road stuff you should be moving rather slowly on snow and ice. Especially when the surface is uneven, loss of traction is likely. In snow an inch of downtravel of one tire could result in a spinning tire. So, as a good driver you should engage the locker(s) proactively before wheelspin occurs.
Remember, lockers were designed to prevent wheelspin. All the new electronic stuff is reactive. Wheelspin has to happen first and then the system tries to rescue what’s lost. Manual lockers are 100 times better.
Proper use of lockers off-road provides enough material for a small book - in fact I am working on one. As a good driver you should engage the locker(s) proactively before wheelspin occurs. Especially when the surface is uneven (and isn't that part of the off-road definition?) loss of traction is likely. Depending on your spring set up (long and soft or short and hard) down travel of only one tire could result in two spinning tires - the diagonally opposed tire of the one that encounters downtravel will also move out of the fenderwell and could lose traction as well.
Even the best driver sometimes goofs. Forgets to lock the diff(s). No worries! As long as the spinning tire(s) are not much faster than the slower tire(s) or your rpm are around 1000 with a manual and below 1500 with an automatic you can still safely lock the diff(s).
The diff locks should not be engaged once you leave pavement just to have them on, in case someting could happen during the next hour or so. Lockers need to be used briefly and strategically when needed. A moment here and there should be enough. I sometimes use them only for a second or so - just enough to maintain or regain traction. I switch them off so quickly because I want to maintain maximum steering capability at all times.
Special attention needs to be put on whether to use the front or the rear locker - or both.
The Mercedes axle diff lock is a dog clutch setup with 5 big strong teeth. As long as the difference in speed between both axles sides is not too much, the teeth of the locker will find their way safely into the groves.
At 1000 rpm in first gear low range each tire rotates at 25rpm - so, even if one tire starts rotating faster, it will only be10 or 20 rpm more. Not much. The diff lock mechanism can take that.
However, if you panic when losing traction and step on the gas more (it is most people’s gut reaction) and then engage the diff lock(s) you might do some internal damage.
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