I dont think it's Marek's disease. Try giving her Poultry Nutri Drench, one drop orally twice a day for no more than 5 days. If it's a nutrient deficiency, Poultry Nutri Drench should help.
I wouldnt be feeding them weeds from the garden if I were you. It's best to let them free range and let them pick and eat on their own. I suspect an impacted gizzard with the head bobbing around. Get tomato juice and give it to her orally via a syringe without a needle. That should get things...