(I wonder if these are the muscles causing my lower back/hip pain?) Improve your triangle by accessing the QL "hip hiker" muscle. When you run or walk this muscle helps your hip lift up. This stretch helps with hip pain.
Psoas muscle attaches to your lower spine and to your thigh bone. If it's tight (sitting too much?) it pulls your lower back. It's one of the common causes of lower BACK PAIN! If you can stretch this regularly then your lower back may feel a lot better!
Does your knee rotate in when you do single-leg work (squats, etc)? This is an AWESOME explanation of femoral rotation, which happens to runners, lifters, etc when the glutes are UNDERactive and the hip flexors are OVERactive. A cause for shin splits and