First of all, congratulations are in order to you for not rushing into treating an illness with steroids. Without the ability to do a thorough exam on your dog personally, I can only suggest the possible presence of osteoarthritis. Since she's 6 yrs old, and without knowing her history of previous traumas (hit by car? dog fight? etc) to that area, I'd be inclined to suggest it's in the early stages of this disease. I'd concentrate on examining the hips and knees thoroughly and evaluating range of motion of them. Any decreased range of motion or pain elicited would confirm the presence of OSA or osteoarthritis.
Second, if she hasn't had any routine bloodwork done, I'd recommend this as well. Certain conditions such as anemia, electrolyte abnormalities (Addison's disease), hypothyroidism, and a few others would present with weakness in the hind legs. Barring a normal physical exam and normal bloodwork results, I'd be inclined to lean toward treatment with any of the drugs in the NSAID's group (Non-Steroidal Antiinflammatories) such as Deramaxx or Previcox. A one month trial would be a good idea to evaluate response to pain treatment. If you haven't had any xrays done, now's a good time. Evaluate hips and knees thoroughly. Hope this helped.