Sounds like the typical problem with many automatic transmissions to me. The issue of being parked on an incline and the weight of the car facing downwards won't let you move gears. If you park on an incline regularly, I would apply the parking brake before moving transmission from drive to park to eliminate the stress on the tranny.
Had a similar situation in recent months with my wife's Ford Edge. She was told by a good samaritan (who said he was a mechanic) she needed a tow and probably a transmission. Thankfully she called me. Luckily she didn't have a wall in front of her, I put it in neutral, rolled about 50 feet to get off the incline and all was well. Just a thought.