When I turned the 4WD on my Explorer 4 ltr.

When I turned the 4WD hi on my Explorer 4 ltr SOHC, the right front wheel when turning left from a stop the wheel seems to be sluggish, choppy and a groaning noise was heard. I jacked the vehicle off the ground and engaged 4WD hi to see if all the wheels turned. The right front did not turn while the others turned while in 4WD hi freely. I put the vehicle in reverse and still the right front did not rotate. I turned the engine off and manually rotated theright front wheel and saw the half shaft spin thru the transfer case and I could see the shaft rotate behind the trasnfer case as well. Just for kicks I did the same for the left front and still I could see the half shaft spin through the transfer case and to the shaft behind the transfer case. Is the 4WD working improperly and if so what could be the issue?