I know your post is old and you have long since resolved your problem (or dumped your Ram), but I wanted to chime in to show that yet another person has this problem. I loaded fridge that was .5" too long for the tailgate to close, so I had to leave the tailgate open. I secured it so it wouldn't bounce by putting a sheet of plywood under the frige extending onto the tailgate. The truck shifts itself into 1st gear so I am prevented from delivering my fridge - or driving to the dealer. I'm convinced there is nothing wrong with my 2019 Longhorn, which has 11k miles. This is just bad engineering. If I remove the fridge I cannot duplicate the issue exactly, but if I don't remove it, I can't drive. Yes I search the OM and this is not a safety feature. I have no intention of going through all the nonsense that AI says to check. This is a specific RAM defect. When I go to the 5th Gen Ram Forum I get those stupid pop-up virus warnings, so I have written that site off.