Sound on the lunge lame under saddle?
So I have an 11 yo WB gelding that I was riding and preparing for the eventing show season. Never had any issues with lameness and things were going fine and dandy until I suffered a terrible accident (not on horseback!). After about 4-5 months of my own rehabilitation I decided to get back on my horse again and noticed he felt terribly weird. He was off work for the most part and lunged occasionally during the duration of my rehab. Never endured any kind of trauma or accident that anyone can account for.
His stifles began locking up frequently so I had them blistered which solved the problem for the meantime. Had the vet evaluate him as he was sound on the lunge but lame with a rider on his back. His saddle was evaluated by both a prof fitter and the vet and does not seem to be the issue. The vet didn't see anything really indicating pain on ultrasound but we did SI injections anyway. He seemed to improve and his back no longer seemed locked up but he was still not quite right with a rider on his back. Few weeks later, had the vet come out again and she actually got on his back and rode him. Said he felt "neurological" as he was completely uncoordinated and unstable although he did not look as so when being viewed on the ground. He tries hard to do as he's asked but just seems he can't. She suggested chiro and working him in a Pessoa to build his back muscles.
Had another vet come out for a second opinion and do chiro work on him. This time he did flex positive on his RH but she didn't think he seemed too off or seemed neurological. Said he wasn't too bad from a chiropractic standpoint. Did neurological and lab tests which he tested negative. Continued working him on the lunge and over cavalettis for a month. Bulked up nicely through his back and hindquarters and travels sound and poised on the lunge. However...he feels waay worse undersaddle. Hardly able to travel straight and can barely trot. He feels tight and braced and his hind end keeps "giving out."
I'm having a bone scan done on him in a few weeks. I have no idea why he so lame and unstable undersaddle but it seems to be some kind of weight-bearing, compression issue. I don't know why he is lame after having time off but I am both devastated and frustrated especially after having just experienced trauma of my own. Anyone have any ideas what it could be? Anyone experience anything like this? Thanks