I am sorry to hear you are worried about Sandi. Thank you for using Just Answer. My name is Rebecca. I am a veterinarian and will do my best to help.
Cushing's disease, among the other symptoms, can cause joint weakness. Can you tell which joint: shoulder, elbow, wrist, that you think looks out of joint? The wrist is the joint above the foot, that allows the foot to bend backwards (towards her tail), the elbow is above that and when it is bent, the lower part of her leg bends forward and the shoulder is at the top of the leg.
You might consider joint supplements such as fish oil, and gluscoamine. There are diets made that have joint supplements added such as Joint Mobility by Royal Canin (I know that one because my dog, a Corgi relative called a Vallhund, is on it).
An Xray may tell us more about what is going on with these joints.
Let me know if this is helpful and if I have answered your questions.