does the compressor stay on with the locker turned off? instead of pulling your diff out, firstly remove the airline from the diff end and block it off or kink it over. if the compressor still runs its most likely in the airlocker itself. you can even try and listen and feel around the diff breather with the locker in.
you might even have a crook pressure switch, you might have to hook up a pressure guage to check it doesnt go over 100psi. if it goes over it will be the pressure switch.
Don't kink your air line, this will damage it and it will always kink in the same spot again, just when you don't want it to. Air lockers work at less than 10 psi not 100, if it reaches 100, you had better stand back cause she's gonna blow (if it hasn't already).
yes easy I found the main o ring on the compressor was leaking cleaned it up then put some thread paste on . If you turn on the compressor it will pump up the tank on the compressor even before you activate the locker. If no leak found or the pump doesn't come on your problem is after the compressor put soapy water over all connections ,that should be at the solenoid then at the connection to the diff, again if it bubbles it s leaking. However check the hose line it might have rubbing on something that is not so rare that it doesnt happen
My dad just suffered the same problem. It turned out to be as simple as the "O" ring inside the diff. Seems this is a common problem with the ARB lockers.
having never pulled the diff out of my truck ,is the O ring a easy fix or something for dealer to service and I believe stupid O rings go in the same s*#t pile as cerclips.