Ok so a 90 axle would have had CAD and drive flanges at the wheels. Someone added the locking hubs. It may or may not have the CAD locked out already.

As for the method of locking the CAD, the above works but relies on the detent holding it in position. I use a different approach. The fork is held to the shift rail by 2 E clips, I'll push the fork towards the diaphragm, remove the accessible clip, slid the fork away from the diaphragm and reinstall the clip. At this point, it can't disengage unless something really bad happens.

After modification, you'll have to move the collar on the axle shafts to engage both shafts. A screwdriver and the ability to rotate the shafts is needed.

This is easier to do than explain and way easier than swapping shafts.