I've heard about a similer problem before, in fact something along these lines is what I was assuming your post would be about. Many users have reported that the linkage-yoke hits the seat under full compression too. Take the spring off of your shock and cycle the suspension to see what's happening. There have been reports of "flaws" in the way that the suspension works as far as hitting things. It is simply very difficult to design an 8" bike with a straight seat tube(and a reasonable wheelbase), if it were easy you'd see a lot more DH bikes with straight seat tubes.