Rook. You didn' t mention what your specific back problem was. I' ve had lumbar surgery from old parachuting injuries etc. and received disability from Uncle Sam. I engage in a rigorous exercises to maintain what' s left of my back. Having said all that I hunt elk in a very aggressive way at high altitudes.
Some thoughts.
1. Are you involved in some sort of rehab or active effort to strengthen your back?
2. Your pack requirements should correspond directly to what you' re hunting, elk vs. smaller game for example and how you intend to hunt. The more aggressive your style of hunting, how far away from camp you range, do you use horses? etc. will determine the type of pack and quantity of equipment carried.
3. If you don' t have horses and will potentially have to haul out a larger animal by hand, you may want to consider a pack frame like the Cabela' s Alaskan Guide series. You can use it to afix hind quarters, etc and still attach a back pack to it.
4. Several guys I hunt with use fanny packs. They ride on your hips and don' t put any compression forces on your lower spine.
5. Any pack you choose should have comfortable shoulder straps with a chest compression strap (pulls the shoulder straps toward one another and has lifts the straps up a bit off the shoulders).
6. It also should have a good wide waist band that fastens across the bottom of your belly. This will help distribute the load and further take the weight off of the shoulders. The waist band should be on your hips. If it' s up in the small of your back, it' s not properly fitted.
7. It also should have several compartments that allow you to get to specific items without having to unload the entire pack.
I spent my whole adult life humping a ruck in the Infantry. Hope this helps.