I can crack my back by pulling the skin..
After a car accident, I was in physical therapy, and while I was face down on the table, one of the therapists was moving my back and made a comment about my skin being easy to work with. She then grabbed a pinch of skin right over my spine and pulled upward. My spine popped! I asked if she just cracked my back by pulling up the skin. She said yes. Wow. So I tried it myself. It works! I reach around my lower back with both arms and grab my skin on either side of my spine and pull out. I can get one or two solid cracks there. I also reach over my shoulder and grab with one hand between the tops of my shoulder blades and I get one solid crack there.
Also, I found I can do this technique on my elbows! I love that one.
I seem to recall trying this on my knees and I don't remember it being successful.