thats a toughy. My buddy has that same problem with his 02 Cavy. The J-Body kit from RK caused some problems with waviness under the trunk. What they ended up doin to stop the wave was to weld a bar under the bumper just enough to stop the dip. But i dunno if u can do that with ur kit. If its fiberglass which im assuming it is, it may be kinda funky to work with.
Yeah I'd suggest a 2x4 or something stuck between the bumper and the bumper mount...it looks like yours got squashed and deformed it...