Hi ian
underneath the airbox there should be a small hose take off from the inlet manifold if i remember correctly
To be honest though id convert it to manual lever difflock, i picked up my 2 door rangie which had a centre diff blown, i replaced the box, used difflock on + off in the drive to make sure working ok but when i used it in anger i couldnt get it out off difflock, no matter how much reversing, spinning up a wheel etc i could get it out + centre diff blew
Have a look here at converting it http://forums.lr4x4.com/index.php?showtopic=35985
hope this helps
James