Which are you missing, Slobber Tooth's? I've found, though, that most of the soul gems that are hard to find involve walking off cliffs and finding a hidden area behind something. I just got the second soul gem last night but I can't remember quite where it was. I believe it did involve falling down off to the side of something. Maybe in one of the caves you walk off to the right and find a bunch of platforms below you? Hopefully someone else remembers better.