no reverse

chevy, silerado 3500 4X4, 1996, 150,000mi, 454 auto Transmiission works fine except no reverse, column shifter engages properly, lever arm on trans moves properly. All fluid levels okay. Prior to total failure of reverse vehicle wouud shutter while backing up hill as if was slipping, then would catch, as if low on fluid. Was not the case! Check eng lite on as well. Need to pull code. Electric solenoid of some sort?? Not able to get much help other than usual go to tranny shop. Am capable of doing most repairs, just need help on locatiing tech info and proper troubleshooting info