There is foam behind mine as well as others that i have seen who have cut out the rockers.
Its for NVH reduction (Noise, Vibration, Harshness) from the factory.
99' WJ, 2.5" Rusty's, Bilstein 5100's, JKS TrackBar, Moog SuperTrailBoss
"With enough fabbing, time & $, it's certainly possible.
That being said, it's like boinkin' your sister...just because you can, doesn't mean you should." - Double E