@rleidle
You are pulling with your arms, so you aren't actually getting any power out of your hip extension.
Have you watched Glen Pedlay's series on how to clean? They are really good. Go here and click on the "Clean & Jerk" section on the right.
http://www.californiastrength.com/olwete.html
Related to this, I found a good exercise for helping my hip drive (and therefor relying less on arms) was doing hang clean high pulls (I think that's what they're called...), where you start in the hang position, bend your knees slightly, then bend at the hips, push them way back so you get a big hamstring stretch and the bar comes down to around knee level, then thrust your hips forward. Like a serious hard thrust. Think about the bend at the hips as loading up the hamstrings like a rubber band then letting it snap forward for your hip thrust.
nb: I don't know if that last point is how it actually works from a physiological point of view... I'm just sayin'