when to engage diff locks

Here is my take;

On dry pavement - Never. You’ll lose all ability to steer. Bad!
On pavement with now and ice - whenever you think there is a need to keep the tires from spinning. 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.
It can be done at any speed. No limitations.

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.

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) and your engine rpm are around 1000 you can still safely lock the diff(s).

The Mercedes axle diff lock is a dog cluch setup with big strong teeth. As long as the difference in speed between both sides (see image below) is not to much the teeth will find their way safely into the groves.
At 1000 rpm in first gear low range each tire rotates at 25 rpm - 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.

When to engage lockers - for example on a Mercedes G