Test for Unbalanced Hind Feet
When a horse's hoof is improperly high on the medial side of a hind foot, the result will be soreness in the back. If you suspect back soreness, check the hairline of the hind feet, then push lightly just behind the loin and just to the inside of the hip. If the horse flinches, the imbalance is causing the resulting soreness. Level the foot and the soreness will disappear.
For more on medial/lateral hoof balance enroll in the online course - Bits, Saddle Fitting and Hoof Balance