Vets ALWAYS take "everything" out during a spay. They do not just remove the uterus, but also the ovaries and fallopian tubes. This does not explain your dog's obesity. Your dog was an adolescent when spayed. Dogs that are going to have weight problems (either due to a medical issue or overfeeding/ underexercising) often begin packing on pounds when metabolism slows as the dog approaches adulthood (in other words, shortly after you spayed her).
If your dog is not responding to a weight loss regimen approved by your vet, it's time to seek a second opinion. Any testing that came back as inconclusive should also be done again. If you're near a veterinary school, that would be the best option.
Your dog is also being inadequately exercised. A normal heeler should typically have over an hour of HARD exercise daily, plus more than one long walk daily, not 4-5 walks a week. They are a working breed. Gradually increase her exercise. At her weight and since she is having issues getting around because of it, swimming is a good option as it will take the stress off her joints. Your vet may have other ideas to increase her activity level safely.