Thanks so much for all the well-wishes for Darcy. She is doing much better! After talking to the vets, since Gabriel never got sick (meaning it wasn't the food) we think the problem is arthritis in her back and hips. It seems that she's been hiding the pain for a while, and eventually just started to shut down from it (as bunnies tend to do). Looking back, I can think of things that seemed relatively normal or unimportant at the time, but may have been indications of a problem. We've got her on pain meds, and the vet says that keeping her on them indefinitely is not a problem. She's still got some diarrhea every now and then, and will never really be "well," but for the most part is doing great! She and Gabriel have been playing more in the past couple of days than they have in a long time. It's so good to have my bunnygirl back!