I'm sure I answered a post about this the other week, if you look back through the pages you might find some info.
We had a horse in on rehab livery with this problem. He had work from the chiropractor and the physion lasered the area. The horse was in for a month (I think, can't quite remember now) having the laser treatment and being walked on the walker. we also used a back on track rug on him.
He then went home to continue rehab with some gentle lunging work on an equ ami.
The symptoms which led to diagnosis was that the horse (racehorse) had stopped pushing from behind and had started to pull from infront. He was also very sore over his lumbar/sacrum area. He also had problems in his front feet, possibly related due to him taking less weight behind.
This horse had a bone scan which showed up nothing.
This is what the chiro thought Ronnie might have - but turned out to be PSD. Causes lots of pain over back/sacral area. Lameness can be quite bad, and bad behaviour under saddle is a given. Diagnosis is ultrasound of the ligament (which Ronnie had). It can be quite serious and require a lt of rehab to get them back working ok. I'm not sure what long-term prognosis is.
Please don't tell me one of your two might have this? If they do, you officially have the worst luck. Or you are in the bracket of people who have the worst luck.
This sounds very like our TB now, he is currently undergoing investigation and the vets want to do a bone scan which insurance won't pay for without a diagnosis, need a bone scan to get the diagnosis - catch 22 so I think I am going to have to foot the bill.
I'm pretty sure that for the TB we had in the bone scan showed sweet FA and didn't aid with the diagnosis. The purpose of the bone scan for them was to make sure there was nothing more sinister going on and that it was worthwhile continuing with the treatment.