Well- I have been at my house for 5 years now, and I have yet to see a black widow or an egg casing. We are horse property, so we get plenty of scorpions...so that is a possibility.
TODAYS UPDATE:
Her balance is still off, but she is standing--not just precariously balancing on stiff legs. She is eating and drinking well, and is alert. I have not let her out of the brooder to see just how off-balance she is. Perhaps I'll do that when I clean the coop this weekend. Crop still normal. No discharge from any orafice.
Poop observations: completely normal. Firm greenish poos, no squishiness or strong smell, each with a little white of urea. No worms, blood, or alien lifeforms.
I am still thinking head-bonk, pre-existing condition, or scorpion. The rest of the flock is at 100%.