Keep supplementing niacin (I prefer brewer's yeast because it has other vitamins too). One of my runners had vibrating legs, and would be the first to plop down. He also was the biggest (and he still is, he also assumed the position of alpha male... he's great). Shakiness seems to be associated with growth spurts.
Since then, I've gotten another set of runner ducklings, and no signs of vibrating legs. Two things I did differently for this group than my first group that might have prevented shaky legs:
1. Supplemented niacin earlier on (proactively rather than reactively)
2. Gave them much more room to run around (helps their muscle strength to keep up with growth... do you have them in a brooder still and how large is it? Even daily exercise for an hour or so outside of the brooder may not be enough)
It will probably go away over time, but it's best to do these few things which might help shakiness go away sooner.