I doubt it's Marek's for what it's worth. We've ruled out some common causes of imbalance. That leaves toxic exposure to insecticides or petroleum distillates. They will cause balance issues. Any leaky machinery sitting around? Rodent bait? Have you sprayed insecticide at all lately? Even on something your chickens can't reach that may have become wet with rain and formed a puddle that your chickens may have drunk from?