It is just like Labyy said, both back legs move together. When Lucy started it at 4mo we thought it was her being a cute puppy, it wasn't until she started school that we actually realized something was really wrong with her ( we th0ought she had Pano). Turns out she had OCD in both stifles. Since then she has had surgery and recovered very well, but she still bunny hops and were are told she probably always will.
When a dog gallops, he gets suspendend (lifts off). It might look like a bunny hop to an untrained eye, but the rear legs are lifted off in sequence. The paws stay on the ground for split seconds only (.32 seconds for Greyhound), so it might be hard to catch whether both rear legs lifted up at the same time (bunny hop) or in a sequence (gallop).
If you are worried about your dog's hips, take her to a vet. If you have a camera that can shoot action sequences, you might want to do that and analyze the photos.
Here are some links that break down the single-suspension gallop: