In my practice, I see quite a few similar situations and some are caused by the ITB or Iliotibial Band, but another likely diagnosis is Sacro-iliac joint misalignment/dysfunction- especially when both of the previous posts indicate that the symptoms seem to have shifted, or feel as if they are shifting, from one side to the other.
When you are standing erect in your bare feet, are the right and left pelvic crests (the bone that protrudes on both sides that holds your pants up) even, or is one higher than the other? Any early morning low back pain (even very slight, or just some stiffness) upon rising? If you lie on your back and raise one straight leg, then the other, do they both go up an equal amount, or can you raise one more than the other?
A Chiropractor can really help with ITB or SI joint problems.
Just trying to help.