Is it Marek's? Parasites? Vitamin deficiency? Help! New to chicken keeping.

[FezAndTendi]

I have two 4 month old hens - one Rhode Island Red, one Dark Brahma. Dark Brahma seems smaller than the others, RIR seems a healthy normal size. They do not seem to have control of their legs or feet very well, nor their wings - falling over, sliding into splits, etc. Signs started 11 days ago with the RIR, then two days later with the Brahma. We separated them from our 4 other hens 6 days ago. 4 other hens showing no signs whatsoever. Eating and Drinking happily when they can get themselves to the food and water. Not aware of a specific incident of trauma, etc. Stool is not abnormal - it has never been consistant, sometimes normal, sometimes runny, never bloody. Have been giving them Hypericum and Poly-vi-sol for 6 days. No improvement. They are currently in our basement in a little pen with pine shavings. See attached video of the RIR.

Just looking for any advice/ideas. This is our very first time raising chickens and we are baffled and worried.