Allison transmission trouble

I have a 2005 GMC 2500 Duramax Crew Cab pickup. Lately, whenever I put the truck in reverse, the line under the R will blink and the message center says "shift range inhibited". I've taken the truck to my dealer and they replaced the switch in the steering column. Brought it home and in a couple of days, same thing, no reverse. If I put it into drive, it slams. Took it back and they replaced the transmission control module. After a few more days, same thing. Now the truck is in the garage for another major part change. Has anyone ever had this problem? Sure glad this is happening in the winter. I'd really be mad during camping season and towing. Any thoughts would be appreciated. My dealer doesn't know what to do and GM wants to try all of these parts before they replace a transmission. That hasn't been talked about yet, but I'm getting to the point where I'm going to suggest it real quick.