Busted brake solenoid in your shifter is my guess.
Pushing your brake pedal sends voltage to your parking solenoid and allows the shifter to release from park and down in to neutral, drive, etc... and you cannot shift back down into reverse and park until you've pressed down on your brake pedal again, which makes me think your parking solenoid may be faulty...I did some Google searches and that issue comes up primarily with people not being able to get it _out_ of Park. Not sure why that would keep it from going into Park.
I drove it around in the parking lot again a few hours ago. No problems.
One the one hand I'm just hoping it was a fluke and not a recurring issue. On the other hand it is so dangerous I'll still probably take it to the dealer. Really sucks to have this happen on a newish truck. I hate when I have to take a vehicle to a dealer/mechanic when I cannot reproduce the problem. Starting to feel a little regret at my choice. (Was debating Ford vs Nissan....have always been a Nissan guy....maybe not as much now).