Besides the truck coolant, could they have gotten into any kind of rat poison or toxic substance?
Other than that, I'd check for worms or diarrhea (coccidosis)??
There's also a link somewhere on BYC with a long list of chicken diseases and symptoms. It's not always that helpful cuz then your hen has ALL the symptoms of every disease, but it might help??
Sorry i don't know where it is. Maybe check with CUDA or dlunicorn's site?