Are you able to record a video of your duck showing the symptoms?They didn’t like their house at first, but he seems to like it. If he goes in, the other duck will follow. If they wanna be in there I let them because if there’s anything outside, they‘ll be smart enough to go in (outside the coop in general)
You can upload the video to YouTube (and some other platforms) and then embed it here.
Being unable to preen sounds more like a neck issue than a leg issue.