Yeah, I have a couple screwwed up disks in my upper spine/lower neck & shoulders. Back problems all center on one area, removing weight from the spinal column. Upper back problems means low weights on my shoulders. Lower back problems means lower weights on your shoulders. I am sure you understand. Sooner or later, ALL the weight rests on your hip bone: through the spinal column if you allow your shoulders to carry the weight, or, on your hip belt if you don't.
Relieving the load on the spinal column usually helps. What I would suggest, is a slightly taller bag than technically fits you. This will provide a greater load carrying transfer to your hips without going through your spine. This "loose" fit should help with larger loads. No, I won't try to sell you on a pack. Only you can really fit one, but, fit it "loose" in the shoulders.
Make sure you use a sternum strap. Looser shoulder straps can dislodge easily. A sternum strap should only stabilize the pack with little to no restriction on breathing.
Wider straps will help. A 2.25" strap will have a greater tendency to twist and rock with the pack than a wider strap...say 3.25".
You may not need a different pack. Sometimes, simply readjusting a hip belt may relieve any pressure. If the pack is leaning "in" against your back, you can load weight higher and closer to your shoulders, forcing it to pivit less against your spine and transfer to the belt. Try some different loading techniques. If you need a good pack, start with looking at a 1-2 pound pack. Then modify it exactly to your needs. Like any good prosthesis, it will take carefull work to fit it properly. Your goal is to backpack without your back.
Good internal/external support is likely necessary. You will need good frame work in any case. A stiffer frame will help transfer loads. Don't worry to much about extra weight on the pack if you need it. A taller pack will also ride slightly lower. Not the best, usually, but if it takes weight off your spine, the bears don't care. Often, using gear to stiffen your pack helps for "temperary" loads, like food, water and fuel. Roll your tent to the full length of your pack and wrap it tightly with guy lines. Roll your bag tightly, wrapping it with a length of cord.(You might need a custom dry bag for this.) Slip them into your pack, one on each side. They make good support members for the pack. Minimally, they support themselves, and do not become "dead weight" in your pack. They do something. Again, the goal is to put the weight on your hips.
Hope this helps.