I use this feature frequently when we are in the field to string out tile and I noticed I don't have full function of 4x4. High works fine but I need the front tires too when we get into muddy situations and risk getting stuck.
Physically feels like its in low, just no front wheel assist. That's as plain as I can explain it.
Any thoughts on where to start looking?

 

Probably not your answer, but I had that exact same problem a few years back.....4-lo and no front wheels turning.....come to find out later that day, some jerk had stolen my front drive shaft!!!! Some people, I swear.......