the G lockers are the most complicated turd buckets on the planet...
first, you have an electronic nanny, that only lets you lock them in a certain order, and, is susceptible to electronic failure. not common, but it does happen. there is a blog post out there that tells you what resistor to solder out to allow user selection.
second.1, you have the vacuum system that the nanny is speaking to. the valves can go bad, and do, like all vacuum systems. the vacuum lines supply vacuum to hydraulic actuators in part 3... but first...
second.2, the engine cannot reliably supply enough vacuum to run the whole system, and the center diff lock relys exclusively on vacuum, so there is aux pump in the system...
third, the actuators hydraulic system engage slave cylinders that in turn push a dog toothed sleeve to "lock" the differential. the arm to the sleeve is known to bend, and the teeth round and cause it to slip out of lock under use.
when the vacuum system develops a leak, and it will eventually, you will find that when you engage the "last" (read that as "front" because of the nanny), the system will not have enough vacuum to keep the vacuum dependent center engaged, and it will release. the nanny, in turn, thinks you deliberately opened the center, which is its cue to open the rear, as the order of center, rear, front is strictly enforced by it.
if the aux pump dies, and it eventually will, you may engage all three locks in preparation for a difficult climb, only to find that when the rpm's dip low, so does the vacuum, and all three open up on you again.
both scenarios are just "oh so fun".
there isnt a worse locker system on the planet. the safety and reliability of the G system isnt even in the same ballpark as the simplicity and economy of the ARB's, or toyotas simple electric motor or cables. please. damage to any part of the system in any of those results in you being STUCK locked, not it opening up when you need it on an off camber climb.