You can just teach her to be aware of all her parts. She knows to canter correctly in the front, but somehow along the way she's lost touch with her hind end. lol
A good exersize is to put her on a lunge line, small enough of a circle where you can touch her with the end of a long whip. Start out by touching her everywhere with it, to make sure she isnt a whip shy horse. If she seems okay with the whip, ask for a walk and just gently start to tap her on various, random parts of her body. Tap her inside hock to see if she will reach that leg due to the aid of the whip. If not, you may need to tap harder to get her to re-awaken her senses to the back part of her body. Do that on both legs, teach her that she has 4 legs, not just 2. Send her into trot by tapping that outside hock or wherever you get the best response on her outside leg. You want your timing so that as that outside leg is about to lift, you tap, the leg lifts takes a step, them comes back down, you tap, the leg lifts and takes a step, etc etc etc. That will encourage her to straighten herself so that she can canter correctly behind. It will also get her to engage that old hind end of hers and eventually put her into a whole package.
If she comes back down to walk, allow it, then ask for trot again. Once y ou feel you can easily send her to trot with a tap on that outside leg,you can tap for a bigger trot, and then you can slowly let the lead out and see if that helps her at all. It will take a while, wont be a quick fix so be patient and allow her to find it with plenty of time and patience. This problem wont be fixed by force or by rushing her, you have to teach her how to use herself again.
She will need to build up the appropriate muscles, things will feel weird, so just go with the flow and dont expect too much. You will get there.
In riding, a horse's energy is like a river- guided by the banks but not stopped by them.