Your bumper may vibrate less if you deaden what it is connected to, or what that is connected to etc...
I just finished my entire Titan today, but I think I'm going to use all the extra on the rear plastic door skins.
And then I still have quite a bit of foam to apply, to rear wall and to rear doors.
Used up 3.5 rolls of RAAMAT BXT and about 5 or 6 years of ensulite foam.
Covered Floor with RAAMAT 1 layer up to about 5 layers in a couple spots (mostly 1&2 layers) 1 Layer ensulite foam.
Covered all 4 doors (inside and out 2-3 layers each) (approximately 1/2 roll for the front doors and 2/3 rolls for rear doors. + Ensulite Foam.
Covered back wall with 2 layers of RAAMAT.(Still need to add Foam)
No Woofers to test it out yet, but I have already noticed a large reduction in noise and vibration. Less road noise, can hardly hear my truck idle, and less rattles from components. (virtualy none!)