Give Back On Track hock boots a try. They helped immensely with my older gelding who has arthritis.
How old is she? It's quite hard to tell how to combat stiff hocks without knowing the cause. If they need ossifying, a physiotherapist can give PEMF to speed it along but they won't if you don't know the cause in case it makes it worse.
We are not sure what is causing them but she is done growing
What's causing stiff hocks? Are you waiting for them to ossify?