I'd have to agree that the waist belt on the McHale is the best I've used (the "Sarc" belt). I think it's a combination of high quality materials (evozote foam), a wide belt that wraps well around the hips, and the double buckle system. I was just astounded when I found that I didn't need to tighten my hip belt while hiking. Given your situation, it might be worth the investment. If you don't go that route, I'd at least recommend that you get a pack with a waist belt that really wraps around your hips, and extends to within a couple inches of your belly button. Granite Gear lets you to swap out belts; when I went from a medium to a large on my Nimbus Ozone, it made a big difference (my son uses that pack now).
Not really my business, but if you recently had back surgury, I'd sure take it easy and give your self time to fully recover...