Ok, my duckling had the shakey leg too. It was like her whole leg would vibrate starting with the knee/elbow and then she would kinda sit back on her elbows and then stand back up in a few seconds and start walking around again. I did a few things which helped and it probably took a total of 2-3 weeks, but it did ultimately stop entirely. First I upped the vitamin B. I fed Flock Raiser to begin with sprinkled with brewers' yeast, but I still had one duckling with the shakey leg, so I bought liquid B complex and gave a full dropper full to the beak twice a day. I continued until the bottle was gone, a few weeks.
View attachment 1611868
Also I gave her swim time every day once a day. She seemed to not have any problems in the pool so I think this helped her strengthen her leg. Swimming is easier than walking as they don't need to support so much weight. You mentioned they can't all swim at once, but perhaps you can take your duckling plus one buddy and give them some swim time in warm water indoors. It only has to be a few inches deep. Just enough that they can paddle around and really get clean. I used a kitty litter pan so when the water got gross I could dump it outdoors. This should help with the grooming issue as well.
The third thing was the ducklings got more space. They had really outgrown their brooder and they got more an more time outdoors. This allowed more space and more time for walking around and strengthening those muscles.
Good luck! Let us know how it goes.
Good for you getting the B complex try giving it over a snack so he gets full dose.
