If it's a niacin deficiency, brewer's yeast can help, sprinkled onto food. Some use niacin pills but there are some discussions going on about different kinds of niacin pills, you may wish to search BYC for those topics. Bugs contain niacin, I am told.
For too-fast growth, watching that they get enough nutrition but with fewer calories (if they're getting corn, cut back on that, add more greens, for example) may help.
The larger breeds can have leg problems, I am told, from gaining weight too quickly. I have a small breed so that wasn't an issue for us.
If all other behavior is normal, and swimming (which takes weight off the legs) is normal, I would lean toward thinking that something is causing the leg weakness. If there are other signs of neurological problems (seizures, for example), I'd lean toward a niacin shortage.
My guess is that ducks have different tolerances for niacin deficiency and perhaps other nutritional components. That's why some people are less concerned about niacin than others, perhaps.