Advanced Button Animation

Hey all, just need a bit of help with some animating. I want to achieve an effect where if you had a Flash navigational menu and you rollover a button, a shadow slides behind it. Then if I was to roll over a button on the other end of the menu, that shadow would slide (from where it was previously from) and go behind the other button which I just rolled over...how is this achieved? I'm guessing you have a movie clip and few invisible buttons but have no idea what needs to be done!! Any help??

you would need to have the shadow as a seperate movieclip. When you roll over any button you would then need to trigger a function that took the current _x of the mouse or similar and tween the shadow to that position. Something along those lines anyway