When I see a hen go suddenly "wobbly", losing motor coordination, I immediately suspect a neuro toxin. Since you say you let the flock out to free range, and the hen came back after that in a wobbly condition, you need to try to identify what she might have gotten into.
Some common neuro toxin sources are insect poison, machinery leaking fluids onto the soil, moldy compost or feed, mushrooms and toad stools, peeling paint. If you've been painting and cleaned some brushes in paint thinner and disposed of it on the soil where the chickens later ranged, that created contaminated grit.
Depending on the poison source, it can sometimes be neutalized with activated charcoal (burnt toast) and milk of magnesia and the nerve damage, if not serious, may be treated with B-complex vitamins and selenium.
