Last summer my 63 valiant the reverse button stuck Went and talked to the local transmission man also a mopar man
There is a valve in the valve body int the trans that's called an idiot valve to keep some idgiot from reaching up and pushing the reverse button while driveing 55 mph down the road to keep from stripping the gears this valve was barely stuck in mine didn't want to go into reverse and the button wouldn't engage, pretty easy to fix if you feel comfortable with hand tools.
GET A NEW TRANS PAN GASKET Take the pan off, draing trans fluid then take off filter if equipped also a good time to repace if any doubt, Take down the valve body there will be a small round valve popped up take a rag and wipe off all the fluid sqirt a little oil or penatrating oil on it and work it up and down several time put the valve body back in place and reasmemble, I done it over a year ago myself no problem's since