We have had x rays and the came back normal. Not sure what to do about this problem

No fleas or skin irritation?

It may not be her bones or joints that are bothering her. It could be muscle pain. It also could be neurological.

Consider... why do some dogs chase their tails? The whining she does in the process may be nothing more than the 'excitement of the chase', mixed a little with the frustration of not being able to 'catch up' with whatever she is chasing. (You're positive it's not her tail she is after?).

After all, Heelers are herding dogs, which comes from a fairly strong instinctive urge to... whatever. I have heard of some herders that will to round up the dust balls under a bed. Would it be completely out of the question to think she is simply trying to 'herd' her own rear end?

Just a thought.

How old is she? How long has she been doing this? Could it be related to 'growing pains' or has she put on weight that might be stressing her legs?

How do you react when she does this? Do you give her more attention because you are worried about her or do you leave her alone and make note of what she is doing?

What has the vet said? Has he/she given any thoughts as to what might be going on?