Marek's or poisoning

Friday I caught one of my 7 week old Australorps, Little Jerry, eating green (unripe) lantana berries from a wild lantana that had come up in the yard. I removed the plant immediately. My other Australorp pecked at the leaves once, but other than that, she and my Barred Rocks (same age) seemed completely uninterested in the lantana. I counted the missing berries - it appeared that Jerry had eaten four or five.

An hour or so later, I noticed that Little Jerry was stumbling around like she was drunk. I checked her over, but she seemed fine except for her balance problem. I briefly separated her from the others, but she squawked so loudly that I took her back to the pen for fear that the neighbors would turn me in for chicken abuse.

Little Jerry's condition has stayed the same since Friday. She eats greedily and drinks water normally (it has been somewhat hot, so all the chickens have been drinking quite a bit). Her feathers look as good as feathers ever do on 7-week-old pullets. She can run well, but walks with a limp. She stands most of the time, but will get wobbly and lie down if the wind blows fast or if she one of the others decides to practice flying nearby. None of the others pick on her; one or two will usually go lie down next to her if she rests. She is first or second to the food (organic pullet developer) in the mornings, so I think she's getting enough to eat. Her poops were sort of foamy on Friday, but since then they've been normal.

Twice a day, I've been giving her (and the others, just in case) a mix of water, pullet developer, oats, a tiny bit of molasses, polyvisol, and yogurt. I was adding Pedialyte to their water, but they seemed to drink a bit less, so I stopped.

My concern is that this could be Marek's, not poisoning (although all the chicks were vaccinated at one day old). If so, I'd rather not prolong her suffering. I'm also concerned that if this IS poisoning, it is troublesome that her condition isn't improving.

Any ideas or advice? Is there anything else I should be doing? There are vets that see chickens about 40 mins away, but I'd rather not subject her to the stress if I can help it.