SI problems are a crapshoot, they never seem to heal 100% and almost always cause issues down the track. My TB gelding has SI issues (probably from racing but who knows) and the only thing that made him truly comfortable was reducing his workload. We'd begun playing with some upper level dressage stuff and he was getting ready to step up to the 90-100cm SJ class but it just made him sore and it just wasn't worth it. He hadn't had too many issues before this point so we believe he redid an old injury in his SI when he was out in the paddock, but even after injections and rehab it was still painful and caused issues when trying to bump up to that next level. So, easiest solution was to reduce the work load, stop jumping regularly and he is happy as larry. I'd imagine massage/physio and injections would be the best management plan, perhaps a joint supplement too?
As for whether its hocks or SI, you'd need to get a full lameness exam for that including nerve blocks I'd imagine. Hind end lameness can be really difficult to pinpoint exactly where its coming from and flexions are often not helpful- Its very difficult to isolate and flex one single joint in the hind leg. X-rays might also be useful to see if there are any arthritic changes in the hocks. Either way, you would most likely need a vet to give you a definite answer.
Oh well sh** that doesnt make me happy... My horse is dealing with an inflammated SI ligament and is on stallrest for 2 months, with injections... I was told he'd be 100% after :( ****, looks like that might not be the case then.
OP, my horse has arthritis in the hocks (very light, it doesnt really bother him, but he has a shorter stride behind and he takes a bit longer to warm up at the start of a ride) and he did something to his SI ligament a few weeks back so he's currently on stall rest. He did get the injection. My vet also said physiotherapy was an option, which would be 20 sessions, 10 continuous and then 10 'separate' ones. That was out of my budget and not really justified given the age/workload of my pony. My vet said he would heal fine with 2 months stallrest and injections alone. Physiotherapy might be worth looking into for you. My guy is getting massages every other day as well.
If it flares up again after he's done healing and back into work I will look into semi-retiring him to a barn where he can be out in the pasture, and where I will maybe play with liberty training, and hack him out on the trails on weekends. Maybe the occasional tiny jump. He was/is in work where he jumped twice a week (80-90cm) and was ridden almost daily. At any rate, my guy is around 20 years old, so 'early' retirement makes sense for him, I dont know how old your mare is.
We think he hurt himself when he was spooking at everything on a windy day, he probably made a weird movement when shying to the side and bolting at some point. According to the sonogram he has a fairly thinned ligament anyway, which makes him more prone to injury.