So the vet confirmed my suspicion that there's coccidiosis, but he also said there are uric acid crystals in the fecal that can be caused by high-protein feed.
The pullets are in a transition from flock raiser feed (20% protein) to layer feed (~16%) protein with scratch grains mixed in so the mix right now is about 25% grower, 25% scratch and 50% layer. Nothing terribly high in protein... they have a run and get the occasional handful of mealworms, but nothing excessive.
The vet said it could be bird-specific (I'm thinking maybe breed-specific as this pullet is also an Orpington) but the outlook doesn't seem great.
She's willing to let me syringe water/mush into her beak and eats some on her own. At this point, it seems to be a waiting game if she'll recover as she seems pretty lethargic at the moment.