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Hello, This is Dr. Jess. Not sure if your question was only for Dr. Bruce as you mentioned the name, so if you only wanted to talk to Dr. Bruce, you can let me know and I will remove myself from your question. I just wanted to mention something else you should consider for your dog. One of the more common causes of lameness or limping that moves from one leg to another (we call this shifting leg lameness) is tick borne disease, such as Lyme disease or Ehrlichia. If it is possible that your dog has ever had exposure to ticks you may consider having her tested for these diseases (it is a blood test) before you have x-rays done on her. Just a thought to consider.
I am glad to hear the lyme disease test was negative. Ehrlichia (another tickborne disease) is still a possibility if you live in an area where the disease is present or more common. You can check the geographic distribution here: http://www.cdc.gov/ehrlichiosis/stats/#geography , so if you have found ticks on her and you live in one of the more commonly affected areas, it might be worth testing for that as well (there is no vaccine against it).
How old is your dog? Has she ever had lameness episodes before recently? Do her toenails look normal? Does she have any other abnormal behaviors when these episodes occur?
Based on her age, and the fact that her lameness has shifted legs and comes and goes, my first suspicion would be a condition called panosteitis, sometimes called "growing pains", just like in children. It is more commonly seen in large breed dogs but can be seen in any dog. Most dogs do outgrow the condition and the treatment is basically anti inflammatory pain medication. If the anti inflammatory alone is not enough, it can be combined with secondary pain medications such as Tramadol or Gabapentin. You can find a detailed description from this website which provides veterinary reviewed information: