Kirsten your cat seems to be overweight from the pictures and from what you say here...has your vet checked Luna for Diabetes? She could have diabetic neuropathy, her condition would then be neurological but it can be treated --that would cause the weakness in the hindlegs. Also I'm wondering if she was checked in case she has a blood clot lodged in the main artery, this would block blood supply to the hind legs and cause weakness. And I wondered if you thought of adding potassium to her diet, it helps humans and animals alike. Matter of fact I'm on extra potassium for weakness in my legs and I've seen a huge change.