95% it's sacral.
@LevelHeaded: thanks, but, my back was in a much worse condition (or my form was much worse, or both) 7 years ago. It would hurt on DLs, squats and flexion. Now I can DL, squat, and do weighted flexion (with up to 90 lbs) with no pain whatsoever, neither in training, neither after (I do get a little tingly sometimes, but it seems to be getting better as I progress, not worse)
Also, I dunno if it's not in alignment or "simply" prominent, and, most importantly, I'm getting near a grandiose double BW DL, and my back doesn't hurt neither when testing my 1RM, neither when doing 12 reps @ 75%, 8@85% or 10 SLDLs @75% (75% = 1.5 BW) (mentioning these because they're different animals).
(sometimes, I do get a little pain, but if it is, it's the first few reps of one set, and, overall, in the last two years, my back got from bad to good to better...and now it's going between good and better, but I know that heavy DLs mean you may get some pain once in a while...)
@olifter: Well, I read an article by Mell Siff, I think, it was something like "What is normal?", and about the spine. So s-lysthesis (if you have it) is not a sentence to not deadlifting heavy?