I have a 2003 Chevy 2500hd 6.6l diesel. All gears and shifting were working fine. Pulled out on highway today and had to get on it. When going through gears I heard it slip up and rev as if it slipped out of gear. I slowed down and got off road. It seemed to be stuck in 3rd gear. I tried putting it into reverse and it would not work. The shifter doesn’t even feel like it’s moving into gear when I move into reverse. When I move shifter to Drive, it sounds very rough and it starts off in 3rd gear. The truck will not shift out of this stuck gear, not even go into reverse. Had to put home slowly in that gear range. Need some help, What’s going on?