As someone who has had back issues in the past, I know how bad and debilitating the pain can be.
 SP2 is dead on about an pack with good hip belt. External frame is the weight carrying standard, but a good internal frame pack with hip belt is okay as well. Some MFG, make packs with the hip belt cut for a womens anatomy. By nature, womens hips are wider and meant to carry a load.
Most women lack upper body strength, and a Duluth type pack sans hip belt puts too much weight on the weakest body part. Load the pack so the weight is low and tight to the body, and loosen shoulder straps, so most of the weight is carried across the pelvic girdle. A good sternum strap will keep the pack close to body for optimum load carrying.

Does your wife weight train, or do Yoga/Pilate's? Disc problems that are so bad that they require surgery are something you can't really fix, but you might be able to alleviate some of the pain, by doing some Core work on a Fitness Ball, and by getting into a good stretching program.
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 Believe it or not, much of the tightness in your lower back is caused by tight HAMSTRINGS, keep them stretched out and that will often make a big difference? Also strong Abs will help support a weak/damaged spine.
 When I wake up in the morning, in the woods or at home, I stretch like a cat. Bring one knee, then the other into my chest and slowly exhale and hold. Don't tug or jerk, cold muscles are like frozen rubber bands, they'll pop and you'll pull something. In the warmth of your sleeping back, the slow stretch on each leg will loosen those tight back muscles easily. You can try at home, a good way too start the day.
 Worse thing you can do is roll out of a warm bed/bag, and jump right up when you have back problems.
 Part of the planning for a trip includes or should IMO, getting the body in as good as shape as you can. It's hard too enjoy anything when your hurting.  Your wife sounds like a gamer, so make it a COUPLES thing too work on the Core and Stretching and you'll both enjoy the trip more ;)