Thank you VERY much for all of your advice, everybody! I took her to the vet, and she said that it's most likely either whipworms (which might not have shown up on my visit to the other vet) or a pancreatic deficiency.

We're going to treat her for whipworms and switch her food. She has another appt. scheduled for 3 weeks from now - when she'll be getting her second Lyme and Giardia vaccines. If she has gained weight by then, she'll give her a 2nd round of whipworm treatment. If she hasn't gained she'll do some pancreas testing and maybe thyroid, etc.

She had me switch her to Eukanuba and told me to give her as much as she'll eat every day until her ribs stop showing.

Thanks for the Satin Balls recipe, Wolfsoul - I think I'll try them.

Here are some pictures of how skinny she is right now: