...in my lower back so I can sympathize with the OP. I've tried several packs without much success. I do have a McHale pack and I stripped it down to achieve the lightest weight possible. Get the weight on my hips.
It didn't work for me which is why I'm now going to the other extreme. My back was least effected when I wore what was essentially a large summit stuff bag. No pockets, no frills and all weight carried by my shoulders and upper back.
I'm going back to that approach and getting the lightest pack I can get my hands on, a ZPacks. Maybe it will work, maybe it won't. The back is a finicky thing and, sometimes, it's the strangest movement, such as a twist one way or the other, that will throw it out.
The OP is doing the polar opposite of me. I hope it works out for both of us.