found this...looks like 85-100psi.
ARB:
The ARB Air Locker is a driver-controlled differential lock actuated by pushing a dash-mounted button. In a tenth of a second, compressed air actuates a piston inside the differential which engages a locking gear, instantly locking the differential and directing driving torque to the wheels with maximum traction.  The diff lock is simply disengaged by again pushing the button.  The Air Lockers lock the axles together, providing 100% traction to all wheels, allowing you to safely traverse even the most difficult terrain.

The ARB requires an air source between 85~100 psi.  The air source must be controlled, either via manually operated valves (I have a friend who did this and says he finds it more reliable) or through an electrically actuated air solenoid.  The later is known to fail upon occasion....fact, not myth.  If you have a spare and some tools, it should not be a big deal to put in a new one.

The ARB locker costs around $150~ $200 more per axle than does the Detroit Locker.  In addition to this, you will need some kind of an air system to operate it, along with the appropriate control switches, air solenoids, and air line, etc.  This will add several hundred $$ to the overall project cost.  If you already have an air supply in your vehicle, you can use it along with a pressure regulator and the necessary switches, lines, solenoids, etc.  This will help reduce the cost.

Mike