I can't get far below parallel either, and I don't think many people can (except yogis).
I also feel it's more of a hamstring flexibility issue than lower back, but I may be completely off on this one.
What did you do for lb rehab? I injured my back a few months ago (not badly but enough to need a couple months off from bridging) and I found a protocol that really helped: 1-3 sets each of 50 reps GM and deadlift with a broomstick; increase reps up to 75 until that's easy, then start adding a bit of weight. Your lower back should be going up in flames. I did this every morning for 3 weeks. Forget where I found the link, I think it may have been on this forum or in Jay Cohen's blog.
Chiropractic helped also.