My Tb stallion does the same thing. I started using thin leather collars and light chains on his back ankles along with splint boots. He started using his rear better and also started round more in his hocks. He was just being LAZY. He was checked by a PT multiple times.
If she can round in the hocks over ground poles then unless there is a physical pain then I'd think she's just being lazy. Make sure she isn't sore in the croup and loin areas as well.
Have either an equine physical therapist, vet or equine sports medicine practitioner look at her also.
Is it in all the gaits? Is it on all surfaces? Has a vet examined her? I've seen horses drag their (hind) toes in a lazy walk to the point of looking like a large snake had slithered through the area's sand, yet pick up their feet nicely in trot. I've also seen a schoolie dragging their hind toes when ridden in a pokey trot with a novice rider (but would lift their feet nicely for more advanced riders). However, I would be a touch more concerned if it happens in an enegetic walk and trot, most rides, and on most surfaces. The hind limbs tend to be more affected by certain medical conditions, and it may be worth having a vet do a work up, including a neurological assessment? Might be alleviated through certain types of exercise combined with balanced feed (vitamins and minerals, esp vitamin E)
Sounds like your mare is doing one of two things A) being lazy and ONLY moving when necessary due to underdeveloped muscles, or B) as Lynn said, it could be physical and I 100% agree that a chiropractor could be necessary. HOWEVER, those bones will slip right back out of place and your problem will return and you'l be on a self-causing cycle. Try a pessoa ( in A LARGE circle on the lunge line), hill work (up hills at a slower pace[walk/trot] will build muscle. Cantering up hills can lead to sore muscles easier.) i would suggest a mix of slightly raised, more raised, and flat on the ground cavaletti/poles. the variation will help your mare develop a keen mind and the physical work will develop her muscles. hips and legs are heavy, horses need a well-developed hindquarters to support themselves. (don't be afraid to let her have a day off now and then from muscle building. horses get sore too!)
No I haven't, I honestly just thought she was lazy. One of her fronts I excuse the dragging for since she drops her shoulder a lot (a habit she's picked up on since the track) it's her back end that drags the most though