I have a horse who does that (he is retired) - he had stifle problems.
For one, kudos to your barn owner for noticing this. It’s unfortunate how things like this can oftentimes slip through the cracks. She deserves a hug and a round of applause. 
Secondly, echoing everyone else, I wouldn’t say going to further corrective shoeing without films to see what exactly is happening is the best bet. Consult with a good chiro that you trust, see if there are any stretches or exercises that you can add to relieve tightness and/or encourage strength. Backing exercises are great for stifles.
Keep us posted on how it goes
[QUOTE=OrangeAmbition;8722445]
Thanks again for the feedback. I trust this vet implicitly. We work together well and if she says that given the utter lack of clinical findings we don’t need to undertake more tests at this stage, I believe her. Just to be clear, the vet did a full physical exam of her back and hind legs, not just flexions. She also watched her go. She and I are both very happy with the farrier’s work, so while we both disagreed with the wedge pad suggestion, he will not be thrown out with the proverbial bath water.
I also trust my horse, who is subtle but clear if she is at all uncomfortable in her job (I have several times picked up subclinical issues before the vet could) and she is totally sound under saddle (no tail to one side, no issues with one lead. I think I mentioned in my op that we had been working on the changes around the time this started and she definitely finds it harder to come through behind going left to right, but that’s it). i am going to take reasonable measures to get to the root of this, but I’m not going on a wild goose chase beyond that, unless their becomes a reason to do so. So I definitely appreciate people’s experiences, but at this stage I have no reason to second guess the professionals who have actually laid hands on her. It seems pretty reasonable to think that she has some muscle weakness (probably reflecting mine) or a bit of instability in her si that are causing the behaviour we are seeing. We are going to start monitoring whether it corresponds to hard work or cycles or anything.[/QUOTE]
Holy halfpass, OP!
I’ll bet your vet(s), farrier, horse therapist(s), coach, and BO/BM (if you board) love you!
You’ve observed a change in your horse’s behavior,
had it evaluated by appropriate professionals,
considered and evaluated their input,
and are implementing a plan based on the relevant findings.
I’ll bet your horse loves you, too!
I’d be very pleased to have you as a boarder!
