1992 Ford Ranger rear axle housing vent

Hi,
I discovered the rear axle housing vent hanging by the tube next to the axle. It appears to have come out somehow, but I can't understand how, as it is still connected to the rubber vent tube which is still clipped to the frame. It could not have unscrewed.
The vent fitting appears to be a different size than the threads in the axle. Is there another piece that screws into the housing and then the fitting screws into that?
Also, it seems that these parts are no longer availabel from Ford. Any idea where I can locate them.
This is really weird, as I have had the truck since '94 and cannot understand how it could have come loose.