Are you sure you have this problem with a framed backpack? You mentioned carrying 20+lbs with a day-pack. Personally, that sounds pretty rough to me without asymmetric bone structures.

With a well fitting framed pack, you transfer the weight to the hips and the shoulder straps keep the pack centered. Most of the shoulder strap load should be on your chest. People often have a gap between there shoulders and shoulder straps when they cinch down the load lifters.