I've had it much worse than you... it could be a bunch of things: irritated SI joint or a piriformis mini-rupture or inflammation etc.
If it doesn't go below the knee it should be a hernia.
Sometimes if you have a rupture muscle you don't know about and that inflammation inflames the sciatic nerve as well, then you might get this. At least that's what I believe.
What about doing just the negative part of the GHR in higher volumes just to the point where you can perfectly maintain form, not necessarily all the way out in ROM?
Basically the same thing as doing negative chinups.