I have very similar pain. I put up with it because it was tolerable, and then one day it was excruciating - I couldn't even lift my left leg up to walk if I had been sitting for any length of time.
The midwife recommended I go to an Osteopath (who is experienced with pregnant women) at 37 weeks and she made an adjustment to my left hip, saying that while pelvic pain is to be expected, I had pinched a nerve in my butt which also ran down the back of my thigh and into my knee. Sitting on the treatment bed, it was obvious that my left leg was shorter than my right - a good indication that it had twisted and was pulling up.
The adjustment gave me a heap of relief. Yes, it's still sore but it is at least tolerable. She also recommended I don't do anything like twisting at the waist (like putting groceries into the car without actually turning my feet as well, or getting out of bed without putting both feet on the floor first), as this will aggravate the pain.
I'm pretty sure an experienced chiropractor or osteopath will be able to give you at least some relief.