you will not believe this but. YOUR exact issues were experienced by my neighbors 2010 RL which I saw him chugging along on the grand central parkway last night.
flashing D light and illuminated CEL doesnt sound promising.
He was able to limp home ( 25-30 miles ) but he mentioned he kept having to turn the car on /off.

 

Got the car back this friday, and the paper work said that it was an internal transmission and torque converter failure.

Car runs great now, wow. Even better then before. Kinda iffy about changing the fluid to the 6qts D4 and 3qts typeF. My dad always said if it aint broke, don't fix it....I'd hate for it to start acting funny on me.

Anyone care to input their .02 cents...?

 

^ i just replaced my tranny at 69K under warranty for the same type of failure. I already have a case of racing atf ready to go in after another 800 miles or so.