Thanks realMakc for the reply. I posted this on another forum also and got a great reply. Here is the reply that worked for me in-case anyone else needs this.
convert the mouse position to a percentage
flip percentage
multiply percentage by the distance between both lines (considering registration point)
Code:
//left and right lines, remember these must be relative to the registration point of the box
var lft:Number = 50;
var rght:Number = 550;
//this assumes the stage is aligned "TL"
var per:Number = stage.mouseX / stage.stageWidth;
per = 1 - per;//flip it because you're flipping the direction 0.5 will still yield 0.5
var ix:Number = lft + (rght - lft) * per;
ix = Math.min( rght, Math.max( lft, ix ) );
myBox.x = ix;
Thanks again for the help.