Sadly there is no recognised treatment for Marek's.
I think what is confusing us is that you are saying that chickens are dying and have broken legs and wings and necks. Broken bones suggest severe trauma usually caused by predator or injury. If your run is secure, then that rules out predators. Having more than one bird injure themselves to such a degree is unlikely. Are you sure they have broken bones or do they just look like they are broken and are you culling them as a result of these perceived injuries or are they dying?
The information you are supplying isn't very easy to follow which is why we are making such varied suggestions as to the cause.
I'm pretty sure that worms will not result in birds with broken legs, so I'm not sure why you think deworming them might help?
Is this problem just affecting young birds or are the older ones also having problems.
There is no recognised treatment for Marek's Disease. It is very widespread and easily contracted... it is caused by a virus that spreads via dander dust. It often first appears as lameness and or wing/neck paralysis and can deteriorate quite quickly or spontaneously get better, although infected birds are carriers for life and may well have secondary attacks which are more likely to prove fatal. Some deaths occur suddenly without any obvious symptoms. Some strains of the virus affect the eyes. Some infected birds develop tumours which can be attached to the organs, muscles or skin as well as inflammation of the nerves which is what causes the lameness. Lame birds will often be found lying on their sides with one leg forward and one back. Some strains are very aggressive and kill a high percentage of the flock, others like the one I have in my flock are thankfully less severe. The virus is very difficult to eradicate from your property once it arrives. Usually young birds between 8 weeks and 12 months are affected.
There are several informative threads about Marek's on this forum if you think that's what it is and need to know more.