David, during my physical therapy after my bike incident, my therapist tried a TENS unit on my left leg and upper thigh, hoping to both stimulate areas where nerve bundles were severed and also reduce the resulting nerve pain associated with limb loss. In my honest and most humble opinion, after several sessions of therapy, I saw no difference in either pain reduction or mobility (and the therapist also concluded it wasn't working in my case, but he wanted to at least try it, JIC). I can't comment on use for other body areas such as the back, it may work for you.
I do wish you luck, because I understand chronic pain issues.