I have a 2014 2500HD Silverado with manual shift 4 wheel drive. I can not shift into 4 wheel at speeds above 10 mile per hour, most of the time. It is difficult to shift even at a stop. I have had the truck back to the dealer twice with no improvement. Any suggestions on what maybe wrong. This is my third truck with a manual shift for the 4 wheel drive and have never had a problem with the other two.