The thing I think about osteopaths is that they are either brilliant or crap, with no inbetween ones. physios tend to fdo more training I think (sorry any osteopaths out there if I'm wrong) and the horse ones do have to qualify on humans before they can learn horses so they generally have a very good knowledge base before they get anywhere near a horse. I think that's good because horses are fussy little animals and need all sorts of care taking with them. One wrong move can set you back for ages, as I'm sure we all know from experience.
On the subject of your horse Zingy, it does sound to me like he's out of work and out of the habit of being good! He seems to get better when schooled right? well there's your answer. It also sounds like he's thinking about it when you get on, rather than trying to throw you off, he's just weighing it up. i know a very lazy horse like that who definitely has no physical problems, but the fitter he gets, the worse he is. He took my friend on a right jaunt last week before chucking her in a ditch and cantering triumphantly round the field, all because she wanted him to change the rein!
In addition, I'd expect most horses to buck after box rest, and I think it's ok for them to have tense muscles after resting for that long. They get stressed and frustrated, so will tense up when they get out!
I agree with wally that if he always hated being shod he may have a leg or foot problem, so ask your farrier to have a think about it, and maybe your vet. Even if your vet thinks there's nothing wrong, if YOU think he isn't right you can ask for referral to a specialist centre, like Leahurst (which is close to you) and they may be able to find something your vet can't. This can be expensive though, so if he's not insured you might not want to do that.