I would also supplement with a broad spectrum vitamin twice a week, and 100 mg of niacin per gallon of water for ten weeks. This is not a for-sure-certain suggestion, but niacin does help neurological development and leg problems in many ducklings.
I would soon let her spend a little very closely supervised time in warm (85F) water, in hopes it may relax some muscles and tendons, if that is part of the problem. She doesn't have oil on her feathers, but five minutes at a time ought not be a problem. You probably will want to dry her off well before putting her back in the clean, dry brooder.
Update: Forest (Gump) is doing pretty well. He actually started to walk a bit today... It was very exciting to witness. I just need to figure out how to get video on here.