It is tough to tell by a picture as depending on the horse's conformation they will all move differently. A video is definitely more useful in gait analysis as far as for lameness.
However the other symptoms you have described (dragging toes, sore back, etc..) all describe classic hock soreness. She may well also have something going on in the stifle, either independent of the hocks or caused by the hock soreness.
Were it my horse I would have the vet out to palpate the legs, do basic flexion tests, possibly some nerve blocking (whatever the vet felt is necessary to isolate the problem area) and then move to x-rays to pinpoint the location and cause of pain. From that point a course of action could be discussed and depending on the findings could include everything from a complete lifestyle change for the horse or some minor injections.
Joint injections can work very well to alleviate pain and swelling in the joint, but ONLY IF they are done in the needed area (ie not just pincushion the horse everywhere for no reason - one well placed injection based on x-ray findings is better than 5 "shot in the dark" injections), if they are done with the needed drug (ie Legend versus corticosteroids, which kind of corticosteroid, etc..) and if the after care is done correctly (the horse MUST be confined in order for the drug to work for a few days).
I think, based on what you have said, that there is likely some moderate to severe hock arthritis (spavin). If you are "lucky" it will be in the lower, fusible joints of the hock (bone spavin) and the hock can be assisted in fusing with some injections or surgery, or simply left to fuse on their own and likely won't cause that much of an issue after fusing. If you aren't so lucky, the arthritis will be higher up in the hock (bog spavin) and you're going to have some tough decisions on whether to treat (lots of joint injections, regular maintenance with Adequan and Legend, etc..) or to retire the horse.
Your other option is "The Ostrich" which is what a lot of people opt for. Get the farrier to put hind shoes on the horse so she doesn't rub her toes off and just keep riding and never even call the vet. Eventually she'll be so sore she keeps bucking you off, at which point you can sell the horse with no known medical issues and you're scot free.
But good luck and as far as treatment options, know that it's going to take a large chunk out of your wallet to treat regardless of what's wrong. So decide now a price limit and after that figure out whether you're going to "Ostrich" or re-home/retire when you hit the "vet bill limit".
They say money doesn't buy happiness -- well happiness doesn't buy horses!