Pekins, being meat birds, are especially predisposed to developing leg problems such as sprains, soft tissue trauma, or infections of the foot. What you're seeing could be due to injury, or an infection even if the skin appears intact. Mycoplasma Synvioe, gout, and viral arthritis are other culprits, but are rather unlikely.
If she were mine, I would treat for the most likely cause, and that would be an injury. For an injury, making a pen for the duck to reduce movement, as well as provide easier accessibility to feed/ and water is often quite beneficial. Additionally, you can soak her legs in Epsom salts every few days, and try to encourage her to get in plain deep cool water about every day to exercise her legs. B vitamins, especially niacin, can be helpful as they play a large role in proper bone development.
With doing that for a week or two, and you still don't see improvement, I would go onto suggest she may have an infection of the hock joint. A simple poke through the skin with a fine object can result in bacterial invasion into the tissue. In that case, antibiotics would be indicated.:
https://allbirdproducts.com/products/baytril-10
