its grinding when in two wheel drive

I have a 2000 tahoe and 4 wheel drive works fine no problems when I put it in 2 wheel drive grinds really bad and wont move any one know what could be wrong? when it is put in four wheel goes fine no noise two wheel grinds and will not move at all I was thinking four wheel drive actuator also put it in neutral and tried to turn drive shaft by hand and it turns a little both ways then binds up i thought rear end or the four wheel drive actuator please help me