I have an 03 Duramax. Have been getting the error message "Range Shift Inhibited" when I go into reverse, have to put it back in park until it decides to go into gear. I have had the tranny serviced and replaced the NSBU switch. It is still giving the error "Range Shift Inhibited" and my tranny slips randomly. When put on a scanner it shows no codes. This has been going on for a few months & have had it to several shops that specialize in Alison transmissions- they are baffled. The tranny is mechanically sound- can anybody help?!?!?!