Broken sprag
I had same fault when i first got my truck. I pulled the transfer box out, removed the front cover were the sprag clutches are, they were gummed up with old oil, i took them out , washed them in parrafin, stoned any small burrs of them, put it all back together and its worked fine since.
You could try draining oil and giving case agood wash out with degreaser, it might do the trick. It would save pulling box out.
I don,t know if the front cover will come off with the box in place, but as Gerhard says check the linkage first, jack a front wheel upand push operating arm in witha bar, you should only be able to turn wheel one way.Best of luck. I just swapped my sprag box for air shift last week.
Best of luck