Hi Babarella...welcome to the kula!
First on your question re upper back pain. Wow...it is really hard to determine what it may be related to from a distance. Have you had a health care person check you out? There could be so many causes. When did it start, were you doing something that may have caused it, ... all sorts of questions spring to mind. Does it feel like a pulled muscle, is it dull, achy, sharp, burning? You see how difficult it is to diagnose remotely.
You asked if the Kidney line may be a cause. Possibly, the body is so interconnected, but I would personally doubt it. As to how to relieve the pain, again it depends on the cause. Generally just moving an area helps to bring energy to the region which helps to make it better. But if the tissues are damaged the first stage is usually to completely rest it first. Then after some rest, and after some of the initial swelling has gone away and you can move a little without any pain, then you can begin to move. Strengthening and opening this area would be the third stage of recovery. Gentle back bends, cat's breath and easy cobras will help the muscles. Gentle sphinx and butterfly poses may help the deeper tissues.
I would suggest though that you be very skeptical of any "medical" advise delivered over the internet
Go see someone who can work with you directly.
Regarding your other questions: Yes Yin Yoga will definitely unblock the body. In yoga you will hear of knots in our energy lines called grantis (sort of pronounced "gruntees"). You don't want grantis! Yin Yoga can help to reduce these blockages to the flow of chi or prana. Will they come back? Maybe ... it all depends. If you keep doing what you have always done you will probably get what you have always gotten. But adding Yin Yoga means you aren't doing what you have always done.
The blockages come from many sources: immobilization of a joint (such as when the body is broken and forced to be immobile for a while), a sedantary life style (the couch potato syndrome), aging, etc ... We can't do much about the rolling of the years but we can delay the inevitable shrink-wrapping of the joints, the fixation and degeneration that accompanies aging. Keeping active, keeping the body (and especially the joints) open will reduce and delay the blockages.
Why not just try it for thirty of forty years and see what happens?
namaste
Bernie