I'm 6'4", and have that problem on bicycles, but not MX'in. One fast remedy might be to try and move around on the bike more. That is, when you move your butt back, hunch your back. Then, when you get on the tank, extend.
I wear out my quads first, and it's not strength that I'm lacking (I'm 220 lbs and squat about 375).
For a long term cure, I think you would first benefit from stretching (playing all kinds of ball and racquet games helps my mobility, too). For power, I like to do just plain hyperextensions (lay down on your stomach and lift arms and legs simultaneously for a simple version) and then deadlifts (stiff-legged for intense hamstring workout, normal for great overall strength) could help.
Take it easy on the deadlifts and make sure you maintain proper form if you haven't trained much with weights.