weird this issue is popping up frequently now it seems. I just did duals on my sons truck and was having the same problem. I found the only way to avoid it was to shift the rear case in to low first and then shift crawl box to go lower. its kind of a dumb issue for me but thats what I found works. when it got stuck, I would have to remove the shifter base and shift the rails with a screw driver. not sure exactly what is causing it but i know what to do to avoid it