That's a tough one Dave. I definitely think getting away from a messenger bag is a step in the right direction, but a traditional book bag has some disadvantages compared to a hiking/camping type backpack when it comes to carrying a load. You've already mentioned the hip belt, which is probably the most important different, but there's also the fact that they lack load lifters to take the weight of the strap off of your shoulder. That's partly because packs made for books are typically less tall than ones for camping, so there's nothing up high for the load lifter to attach to. They're not really made for comfortably carrying a load for any length of time, because that's not their typical use. Usually, they're only worn for 10 minutes or so between classes or from the car to class and back.

Having followed this forum for some years, I've seen the the more experienced members give the same advice to less experienced pack shoppers countless times. I don't see why this advice would be any different with a book bag, so here goes. Fit is everything, and it must work with your intended load. Either buy multiple bags from a store with a good return policy and try them each, loaded, for several hours at home and return all but the best, or find a store that will let you bring your load to them and try it in the store. Obviously wearing them for hours in the store is not practical, but you should try it for a good 5 - 10 minutes. I'm not sure the traditional hours long trial period really applies in your case anyway. If you plan to return packs, make sure you keep them clean and dry.

In any case, I hope you find the perfect pack and don't have to deal with bone/nerve issues any more. Good luck.