If your cat has always had this problem, she probably had a problem during the birthing process. If this happened after you adopted her, she may have fallen off of something and landed wrong causing pinched nerves in her back. You need to try to remember when you first noticed this symptom and what she might have been doing at that time - how active, etc.
If your cat is overweight, this will definitely add to her discomfort. Did either vet take x-rays to see if her spine is out of alignment?
It might also be a deficiency in her diet that's leading to her becoming weak. I would suggest that you try a holistic vet since the traditional vets haven't helped you.
In the meantime, run your fingers along her spine from the top of her neck to the tip of her tail to see if she shows any signs of discomfort. You can also feel if her spine has an unusual knot or bump that doesn't feel like the rest of her spine. If she shows discomfort, a holistic vet (trained in this specialty) will be able to realign her spine and hopefully give her better mobility.
Also, please make sure that your cat is eating organic high protein food - read the label. The link below to petfoodtracker will give you some options and keep you informed about the recall. Today's basic pet foods are loaded with growth hormones, pesticides and stuff that isn't fit for humans - like diseased animals, animal parts that people can't digest, etc. Cats need meat for proper nutrition as they are carnivores. Lack of meat in their diet can lead to kidney problems and more.
Current Safe Pet Foods List supplied by PETA.org:
Active Life Pet Products
Amoré Pet Services, Inc.
Animal Food Services
Artemis Pet Food— NEW
Azmira Holistic Animal Care
Burns Pet Health, Inc.
CountryPet Pet Food
Dry Fork Milling Co.
Dynamite Marketing, Inc.
Evanger's Dog and Cat Food Co., Inc.
Evolution Diet, Inc. (entirely vegan)
Good Dog Foods, Inc.
Halo, Purely for Pets
Happy Dog Food
Harbingers of a New Age (entirely vegan)
The Honest Kitchen
Know Better Dog Food
Kumpi Pet Foods
Natural Balance Pet Foods, Inc. (has vegan options)
Natural Life Pet Products, Inc. (has vegan options)
Newman's Own Organics—NEW
PetGuard (has vegan options)
Pied Piper Pet & Wildlife
1-866-893-4006 (Outside Ottawa-Outaouais)
Raw Advantage, Inc.
Rocky Mountain Natural Products
1-877-768-6788 (Eastern U.S.)
1-800-665-5521 (Western U.S.)
Sauder Feeds, Inc.
Stella & Chewy's LLC—NEW
Timberwolf Organics, Inc.
V-dogfood LLC (entirely vegan)—NEW
Veterinary Nutritional Formula
Wow-Bow Distributors Ltd. (has vegan options)
Wysong Professional Diets (has vegan options)