I would have a good long look at shoulder pain. Mine was exactly the same. He was VERY naughty for his fronts but not his backs, and my farrier (who is old and very experienced) said if he was being naughty, he would be the same front and back.
It was actually a shoulder issue that was causing Tobes pain. I had had a back lady to look at him and frankly she was not much good, and didn't identify it as a particular problem. The lady my farrier recommended took one look at him and said he had a real issue, and after a lot of hard work on my part, the issue has resolved.
Tobes if he was a person would suffer from ADHT, he lives in the moment and bores quickly of lots of things, but he was so good last time my farrier suggested I should call the vet because there was clearly something very wrong with him!
We have also found that Tobes although he has excellent feet - round and strong, has very big frogs. We therefore take one front off at a time so he always stands on one shod front foot, and this has helped enormously.
Horses generally are very generous creatures who put up with a lot. If Sox is kicking off about being shod, I would lay money on it that something, somewhere is hurting.
A haynet for us is a must as well. It helps to alleviate the boredom!! As is a punctual farrier!
Edited to add, that in my opinion, which I am sure a lot will disagree with (!), a good back person is far more likely to pick up muscular issues than a vet. I would get a back person out to save more hassle - but make sure you get someone who knows what they are doing, and has more than one string to their bow training wise.