So without having extremely good 3D video + high frame rates, some things I see. Keep in mind this is more from a biomechanics perspective and not a technique perspective, though I believe your tennis will improve as a result of improving your biomechanics.
1) Overall, you hit very stiff. Just watching your posture, you have a very stiff and rounded back. I suspect given your statements of "having a very strong back" is that you have quite a bit of muscle mass that might be limiting your range of motion.
It seems like when you serve, you are dropping your body to allow your arm to reach vertical. I actually suspect THIS is your source of pain. I don't know how your hip was feeling at the time of this video, but it looks like your hip collapses fairly quickly when you load it up on your serve. You aren't getting a lot of push off from your right leg.
2) I don't necessarily see anything in the lower part of your body when you hit open/neutral stance groundstrokes. Again, you just look pretty stiff overall. I want to say I do feel like I see some right sided scoliosis, but it might just be because that's what I want to see. It's not assessed very well if people are moving and w/ clothes on. You can bend over and touch your toes and ask a friend to look at how your spine tracks. I do worry more about your shoulder as I think your stiffness is making you really arm the ball.
I would be curious to see what your range of motion is like.