Shift range inhibitor???

Hey guys, need some help with my cousins truck. Its an 8.1l with the allison. He couldn't remember if it said inhibitor or inhibited... But I've never heard of this and he was just telling me tonight that he was driving down the hiway and it felt as if it just went into neutral. He said he tried to put it in gear and it wouldnt move in any of them. Finally he said he got it into 1st and got it a mile down the road then nothing. He also said whatever gear you try to select the bar under the gear on the dash will blink. So anyone know whats going on and how to fix it or how to get a lead on whats going on??

Thanks