Backing up - strange action - ideas please?

[jill]

A 16 year old 15hh cob mare who is on retirement livery (owner abroad) has a slightly strange action when we ask her to back up, both in hand and ridden. She sort of stretches one hind leg behind before putting the foot down toe first.
When I first spotted it I thought she was unsure of her footing so did a bit more, and she got worse. Forwards is no problem, apparently perfectly sound, and if she does no backing up it improves slightly.
Her owner doesn't want the expense of a vets visit, and I tend to agree as it isn't a problem, she isn't in work and if she were it would just be a bit of hacking.
I wondered if it could be a sign of a long since slipped stifle - it doesn't appear to be her hock or fetlock joint. I haven't done a flexion test, but her hock appears to be fine. What does anyone think? I'm really more curious than wanting to do anything about it.

[mags]

Shiverer??Does she hold her tail out behind her when she does it?

[kayron]

or stringhalt

[Donnalex]

Sounds like a shiverer, thats why vets reverse them when inspecting them.

[jill]

Not noticed her holding out her tail and there is no tremor - any other ideas from the late shift??