You don’t say how much sway you get. Are you just getting a little sway side to side,because the load lifters create about a finger of space between your shoulder and shoulder strap? Or is it serious movement,throwing you off balance at every step and twisting the waist belt? A slight amount of movement is unavoidable,with emphasis on “slight” - just enough to avoid straight jacket fit.

I agree with Mouse: “REI expertise” is often an oxymoron. One “expert” tried to convince me I needed a size large pack instead of the medium I thought I needed. When I asked to try the medium, I found out they had no medium left; his “expert” fitting was solely based on “you need large because I’m out of medium.”

Take all your gear with you when you go back to REI, and have them help you pack. If they can’t get the pack to work, ask for a refund and go to a local store (Dicks and Cabela’s don’t count.) Talk to staff until you’re confident they can fit you properly. If they’re not willing to fit the pack with your actual gear, you’ve got the wrong store.