If you get it back and the rear-end is still swaying and riding that way the only next things I would think to check would be your rear control arms, rear sway bar mounts and end links, and lateral bar(aka "lateral link" or panhard bar). But I would also think by now your servicing dealer already checked those items.
My rear air compressor went out and the rear-end on my '15 Yukon XL Denali was riding like shit, bouncy, and harsh. There were no error messages on the dash, no service suspension lights, nothing! Like mentioned in a previous post I just grabbed the air bladder portion of the shock and it was never full, always soft, empty feeling.