Agreed with Amiga....maybe even separate the sick drake from the seemingly unaffected ducks....you said the 2 hens didn't seem sick yet right? What kind of housing and bedding is he using? You said the feed is an All-flock? The ducks have a pool? Definitely double check for any dirty water sources that could make them sick...Amiga, I may be on a limb but could a lack of niacin be affecting the legs?
Generally, ducks need more niacin in their diet than chickens for proper leg growth and development, but the only reason I can think it would be deadly is if the legs got so bad the duck couldn't stand to get to food or water so that's a stretch I think...
Um...make sure that there is no debris or chemicals or anything around the birds could injest or choke on....I had a chicken die a few months back from seemingly unkown causes...a similar situation...I put them to bed fine that night, or so I thought, but my hen Ginger didn't make it in to bed, the next morning I found her in a sitting position in the yard, dead. The only reason I could come up with is that she injested a piece of glass or wire and lacerated her gut. I took a magnet through the yard a day later and came up with all kinds of crap so it's very likely.
It'd be strangely coincidental if that happened to 2 ducks, but it seems more likely to me that they are being poisoned by something.
As an immediate solution....Amiga told me this once...you could try getting them to drink some water with charcoal in it. Charcoal is meant to get rid of gut discomfort. I gave some to my Runner Ivy once after she had been throwing up...again that's assuming the drakes were poisoned and it may or may not help, but the good thing is that eating charcoal shouldn't hurt them at all except it can get rid of good gut flora too, so it could be worth a shot. Good gut flora can be replenished with probiotic yogurt or kefir ...they sell caplets of charcoal at Walgreens in the indigestion section. Break one open and mix it in water. I forget the ratio....maybe Amiga will chime back in. I'm sorry about your ducks, I hope something more definitive crops up soon. Keep us posted.