it might be..are your struts and springs at the front any good? or the stock old ones, cos my nice TRD green inserts in my struts acutally colapsed on one the drivers side, and would only come up to about half the height of the other one, then flop straight back in! the nose really did dive under hard braking, and the car felt sloppy all up. so the excess weight transfer might be causeing this. (it sounds like something from gran turismo i know!)
my remidy was custom coilovers, and new gear in the back too..
(it was even noticeable with the wheels i have on the car at the moment 15x9" speedstars with 205/50 rubber on it!)