You've got something communicable making the rounds.  Something in the "very not good" realm of sicknesses.  And if that's the case, the rest of your flock is in serious jeopardy.  Please quarantine the one with the leg issue away from this one, and unfortunately, please send this one off for a necropsy when she passes.  I'm very sorry.
- Where did the two roosters come from, when did you get them, and how old were they?
 
- The same Tractor Supply or many?  What hatchery do they source from?  Does that hatchery vaccinate for Mareks? (Call the stores and ask who the hatchery is, don't take Hoovers as a generic answer, lookup the hatchery and find out whether they vaccinate.)
 
- What did you get in November? Age, source?
 
- The other 20-30, ages and month/year acquired and source please.
 
- Same for this sick one, the other one with the leg issue, and the 4 that have died.
 
- (Basically, list every chicken you've had in the last year, month acquired, age at acquisition, age now, if deceased, what from and when.)  You can group them if they have the same data. 
 
- When did the first one get sick?  Did you get any new chickens, visit anyone else's farm, go to a show, or otherwise do anything that would bring you into contact with another bird (any game, farm, or exotic) in the 4 weeks prior to the first one getting sick?
 
- Any predator attacks where the body was discovered by the other chickens before you?
 
- Last time you wormed them and with what.
 
Just answer each one as you can, don't wait to make a whole post.  You may unknowingly say something that provides a clue.
If it's about the size of fish tank air tubing, it should work.  Maybe 1/4" O.D.