Imo, I would drop some weight off that dog. Any extra weight will aggravate issues like this. I saw that x-rays were done, the hips were checked correct? The most noticeable area that was lame is the left rear hip. Does the dog have problems defecating? Could be a pinched nerve or ruptured disk.  Hansen Type 2 is a problem found in large breeds such as lab's that can cause issues.  http://www.veterinarypartner.com/Content.plx?P=A&A=3223&S=1