Center diff splits the power 50 50 front and rear. Keep all diffs unlocked on dry asphalt. lock the center in snow and ice, and loose surfaces, mud, gravel etc.
Not sure about the need for an air cylinder shutting the IP of. The governor should shut off the fuel anyhow when you take the foot of the throttle. Only to give fuel again just above idle.
Unless you want yo use the exhaust brake below idle rpm there would be no use for that air cylinder .