I'd definitely look and research SI issues. My horse had the same problems, constantly swapped behind at the canter, toe drag and every so slightly off. I had a lot of vet/chrio visits and we focused on slowly building muscle around the area and eventually, a couple months, he had no problem. he occasionally has a bad day, but we just work through it.
Being that high up can be hard to diagnosis at times as well.
Think about what tests you are willing to do and how much you are willing to spend. Are you ok with blood work, x-rays, ultrasound, nuclear scan, spinal tap, MRI, or arthroscopy, depending on what they find? Is general anesthesia something you will ok? Ask how much every recommended test costs and how it helps to pinpoint the diagnosis and affects the treatment plan. You don't want any surprises.