The kind of back pain that jointed stirrups are really going to help is if your back is hurting due to stiffness in your ankles or knees. If you're limping, or favoring, or doing anything else that is making a change in your body mechanics due to ankle or knee issues, that would make your back hurt. And jointed stirrups would help that. I have damage in both ankles, one knee, and a hip from prior athletic injuries, and jointed stirrups make a HUGE difference for me.
I don't think they would help back pain that doesn't come from a problem that starts in the legs, though.
What I *would* suggest, since you say it's come on with a change in your position and a lot more work, is a tube of arnica gel. Can't hurt to try it, and it would help sort out if this is muscular.