I just got done adding sound deadener to the doors and rear wall of my truck. I had heard that it was good to do this to the doors and what not so I did but I didn't expect any major difference in sound or quiet. I am very surprised at the results. The sound is much better. The bass is much cleaner and seems to hit harder. It is also a lot quieter in the truck. I must say that I like it. The job took a few days but it was worth it.
I used about 4 sheets Polymeric Mastic item # 9709T19 from www.mcmaster.com. I put it on the rear wall (behind the rear seat) and on the doors (on the cab facing side of both the inner most and outer most layers of metal). I guess the only bad thing is that I can barley hear my TRD exhaust if the music is above a moderate level. It also seemed to make the noise from the engine compartment more noticeable than the exhaust.