I feel for you all - I have the same back pain since my triple bypass. Also have been searching for an answer (that's how I found this!).
From what I can gather, the pain is a by-product of the surgery, "pain the back following any open heart surgery is extremely common because when we open the middle of the breast bone and retract the bones to reach the heart, we also crack and displace certain ribs on the back side near the spinal bone and some of these fractures do not even unite".
Tylenol or Aleve (lots of it!) seems to be the only thing that helps me.