Somewhere in a "Mine" in Brocken Valley you can find that Crystal Skull!
But the Mine-entrance is "hidden" ... in other words: stay place on a forgotten outpost behind a river ... It's not so far from a 'torture teller' I remember ... so make sure you search
everywhere in Brocken Valley!
I speak for myself (experience): I don't see that Mine at the time I was in Brocken Valley, but later on when it was to late ... (explore with a older savegame and found it) ... and I play futher without completing the quest 'Vigor Mortis'
...
