Pull the locking hub and see what the damage is.
There's no need to get as deep as the bearings.
I found my passenger side would always be locking itself.
I finally swapped to Warn premium hubs.
While they do take a full revolution to engage, I found with the stronger detents they never moved from the 'Free' position.
I suspect that your locking hub has broken internally and is keeping the splines engaged.
Plastic parts have a habit of doing stuff like that.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.