I'm having the same issue with my UN55NU6900 as many others. I have only had this TV for 1.5 years. These is completely ridiculous. The shadow is now taking up the whole right half of the screen. The picture diagnostic does nothing. I have been a long time Samsung customer and can't believe my only option is to pay $200 just to have someone come out and look at it. That doesn't even include whatever parts need to be replaced.
The shadow is always there on my 55in NU6900 regardless of the source or app. I also have the 50in NU6900 that was bought 1 year ago, that one hasn't had any issues, but now im just expecting the same thing to happen one day.