Great videos. I see what you're saying and can see that after going all day she could get worse. Can you grab another duck and check it's legs. Use it to see what the normal range of motion on all of the joints is, then check the gimpy one. Look for any swelling and any joints that don't flex or...
DE doesn't help with anything. :( As for food, I would switch them to some sort of all flock feed and off free choice oyster shell on the side.
Can you post some pictures of her legs and feet? And how about a video of her walking?