Seems like it would be fairly simple (although not without risk, of course) to inject the hocks and see what you have. A trial of hind shoes would also be a simple intervention. I like to do one thing at a time, but in this case I might just try both to give the hocks the best chance of declaring themselves "guilty" or "innocent". If he fails to improve altogether then you have a diagnostic dilemma and a full lameness evaluation would be reasonable. If he responds well to hock injections/shoes, you can experiment with hoof boots behind for riding if you don't want him shod all around full time. And maybe a trial of Pentosan to keep the hocks lubed if he responds well to the injection.
Why is the horse reluctant to load its hind heels? Horse is toe stabbing both hinds, left is worse than right. Left fetlock descent is less than right - indicating compensation on loading that side. Definitely uncomfortable behind, but without hands-on that's all I've got for you.
Just remember that the severity (or lack thereof) of radiographic changes do not equal the severity of clinical disease - so, unless the horse also blocked out to the hocks, you really don't know if that is the source of the lameness, and so you don't know if treating the hocks will help. If you inject the hocks and it doesn't help (because that wasn't the source of the problem), then you will still have to do the comprehensive lameness exam with blocks (and will end up spending more since the injections will be "wasted").
So that's why I said I would start with the exam and blocks! :-)