Do you have access to jack the front end of the truck up so that both wheels are off the ground? If you do, spin the tires with the truck in 2H and then again in 4H. The wheels should spin independent of each other in 2H, but in 4H if you spin one wheel forward, the other wheel should go backward.
There is supposed to be an actuator on the front axle that is supposed to lock the front axle shafts together in 4H and 4L. It could be possible that the actuator is not disengaging all the way, which is where your grinding is coming from in 2H.
Hopefully someone else will come along and give you some other ideas also.