Have you tried rollers on your back.
I have a bunch of them. When I am really locked up tight, I take the 6" dense foam one and put it at the top of my ass crack. The other is 8" and has a 3/8" foam pad around a piece of PVC. This one goes anywhere from the small of my back to the top of my shoulder blades depending on where I need the work. I just then lay on the floor on the rollers with my arms crossed over my chest. Then using my meet I roll back and forth about 6-8" until the muscles start to break down.
It works wonders.
B-