To send a chicken to a lab, do not freeze the body. Double wrap it in plastic and refrigerate it until ready to send it off.You mentioned using ACV in their water, is the container plastic/glass? From with I've read over the years, vinegar has a toxic reaction with metal. I used to put ACV in the water but now ferment feed, however every thing is plastic, glass or wooden spoon.
IF another passes, the best way to find out what's happening is get a necropsy as advised in prior post.