By "crooked" do you mean that the tail turns off to the side? That's what it looks like to me, if she's the one in the middle in the last pic? I would guess she's had some sort of back injury, or even neurological damage, that is causing her to hold her tail that way. I don't think there's much to do about that, to be honest. Best bet would be to take her to a vet for a thorough exam and possibly x-rays if you are concerned she might be in pain. Otherwise if she's eating and acting like a normal, happy duck, I wouldn't be too concerned. Just watch for signs of pain, such as decrease in appetite, behavior changes, lagging behind the other ducks, reluctant to walk or move much, etc.
This is just my initial thought with the pics and what you've described. If this was a new thing I would definitely suggest a vet, but since it's been this way for a year and she seems to be doing ok otherwise, it's not as concerning.
