My Lowe alpine (discontinued long ago) was the best load carrying pack I found. It was 7.5 lbs empty though. The secret was a pack bag that was tall, narrow, and slim for its capacity. It was also critical that the pack bag curved forward over the tops of the shoulders rather than being straight. The big medium stiff belt provided better wrap around than stiff belts and this, in combination with the width, eliminated hot spots on the hip bones with big loads. The lumbar pad was also a key component - it was the piece that kept the hipbelt from sliding down in the back with heavy loads.
The adjustable shoulder strap packs are nice with big loads because you can adjust the torso length to take weight off your shoulders even when the frame sags a little under big loads. This is nicer than just loosening the shoulder straps because it keeps the pack closer to your back so it isn't wobbly. You can do some of this with the load lifter straps on non-adjustable packs, but not all of it.
For uneven shoulders, I recommend a Kelty external frame pack. The shoulder straps can be adjusted for height independently. The Kelty will weigh less than internal frame packs capable of carrying huge loads. Ie: "Only" 5.5 pounds. The external frame packs work well on trails, but not so food for scrambling up rocky areas that are the equivalent to walking up stairs 2 at a time - due to their higher center of gravity. The Kelty worked much better than its competition in terms of hip belt comfort, frame squeak, and durability. The scout shops have a few models, and Bass pro shop stocks them in my area.