My opinion :
I've talked about this with a lady who had back problems like myself and she went to a chiro I went to the other kind, although my back problems were caused by an ill fitting saddle for me and the horse. Its a bad idea to drop one stirrup and it causes negative effects for your balance and back. You should develop a natural Balance with out stirrups first, I have been riding only bareback recently and it has really fixed my back I have no pain now and no pain in the saddle. You need to practice sitting up straight and relaxed when you sit down and riding. Lengthen from your sternum put your shoulders back and relax your lower back try to be aware when your pelvis tilts to one side. Rounding your lower back ever so slightly can strengthen it, if you tense your lower back. Try not to poke your neck out like a turtle and when you ride don't look down all the time. So you need good posture and relaxation in the saddle and sitting at work/school/home. This is gathered from my experience and some other persons.
I realize I ride like this bareback- I would compair it to putting Barbie on a breyer horse.. problem is- my right leg can stretch higher than my left so I compensate by sitting crooked. I found that doing some leg stretches to make my left as pliable as my right has helped fix that problem.
Or are you kinda slumped over at the back- Like humped over? I noticed I do this after a long trailride because my middle back starts to hurt..