there are a couple ways to fix this. you could do it correctly per the info on the main site under problems and fixes i believe. or you can do what i did on the '02 and drill small holes in the plastic shell that's warping upwards, then use black zipties to pull it back down to where it should be, behind and under. i posted pics of it. search my bumper "getting off the reservation" thread. it ain't pretty but it held great and still does. you're probably missing some clips...
Just done mine but and I quote my bumper isnt warped.....the cause is, well Im sure but stand to be corrected, rear cab mountings..... anyhow I removed my bumper and bugger I didnt have time to take pics as time was a factor.
Inside bumper near the middle is two metal half moon support brackets, twist them off with a vice grip.
While the bumper is off you can now adjust the two brackets attached to the chassis but without twisting off them half moon bracket the bumper will not go any lower than it is now.
To remove bumper takes tops 20min, getting it right takes forever.