Selecting Difflock and G - only when stationary?

[Winchweight]

Perhaps a noddy question: I guess stationary is the safest option, but can I change down into G whilst rolling, or select / deselect the difflock whilst moving?

For example if I tackle a hill and realise once moving that I need G or difflock, must I stop or can I quickly select them whilst still rolling?

[Aidan]

can be selected on the fly, G gear has a synchro just like Reverse
only if there is wheel spin across the axle will the difflock actually engage even if selected
you try to learn when you might need it and put it on in advance ideally and then straight off again once clear of the need same with anticipating what gear to be in - G gear can be useful crawling in very very slow moving traffic saves riding the clutch in 1st and going on the brakes all the time like the car drivers do
Only with a decoupler would I say select at rest/as pulling away as there is no synchroniser for that

[Winchweight]

Thanks mate.