I think the first thing I would do is treat for coccidiosis. They sound like they're sick, or heat stressed.
Aside from that, were they in the sun too long? I doesn't sound like Marek's, unless they can't use the leg or legs at all.
It's too late to do anything if its Marek's. I've separated the sick ones , and also did not, and there was no difference, my 10 chicks last summer all died. Once they're exposed, they are exposed.
Vaccinating. There is alot of mis information out there.
Vaccinate all your day old chicks for Marek's or buy them from a hatchery already vaccinated. Quarantine those chicks for 6-12 weeks. (I do 8-10).
Vaccinated birds will not give unvaccinated birds Marek's-from the vaccine. But if vaccinated birds are exposed to Marek's, they can carry it and spread it, but won't die from it Edit: Vaccinated birds will hopefully not die from the virus.
Most unvaccinated adults have resistance as they get older.
Vaccinate no matter how small your flock is. If you have a flock, and add more, you may introduce Marek's to your flock. Or your flock may have it and you will not know. There's no way to tell except by necropsy.
I hope it's something else.