Before tearing into the axle...or AX-L as you call it,
I would get another servo motor from junkyard and see if it is strong enough to get the job done. Weak motors will run but won't have enough power to move the pins in the planetary.
Go to utube and search eaton two speed rear ends and get an education on how they work and what can screw them up.
It is my guess that there is nothing wrong with the gearing...but the servo isn't getting the job done. Not sure what the servo would cost these days but I suspect you can find one on ebay which will include the whole rear end with it. Lots of these old rear ends laying around out there for the picking if you can find one that will fit.
