On the other side of the coin, a good internal-frame model still transfers a lot of weight to the hips, plus will distribute it across the back. So that would work, too. One of the best internal-frame packs for the money is Gregory's Forester ($239; www.gregorypacks.com). For weekend trips, Osprey's Aether 60 ($199; www.ospreypacks.com) is da bomb.
Another thing to do is reduce the load every way you can think of. Don't worry about the pack's weight because in most cases that weight helps keep the frame from sagging and collapsing around your chest. But look into lighter tents, multi-purpose clothing, a light sleeping bagbasically, whatever it takes to get ten pounds out of your "typical" load. These steps alone will make a huge difference.
Also, in time that shoulder will get better. I shattered my collarbone five years ago and still have this grotesque knot of bone on my right shoulder. For a long time, carrying a pack was miserable. But this year it hasn't been a real problem.