R.L.
If you are worried that there is a physical reason for this behavior, then have teh Xrays and other tests done. Or get a second opinion. Otherwise, if you feel there is no physical reason, leave her alone. The more you push it the more she will walk on her knees. If you give her some space and act like it's no big deal she will probably be walking before you know it.
Also if there is anything she likes to do (go to the store, park etc) you could try and give her an incentive by saying that you can't go until she can walk like a big girl.
Good luck!