Thanks for the reply Mark.

I don't think its the diff breather as that one I can see coming off the top of the diff and its a larger diameter hose.

This one is vacuum hose size, coming off the top of the actuator. The FSM also refers to it as a vacuum line, but no indication in the FSM of what it does. I need to trace the other end once it warms up a bit here.

And why oh why did they make this plastic? Older ADD actuator fittings were metal. And I happen to have a spare one of those older style actuators.

I may try and remove the cover and see if I can JB weld a little brass fitting on there. The actuator is over $300 from the dealer and it doesn't look like they can get just the cover.