Harper7 used some stuff called E-dead he is the only one that I know of so far but I have been thinking of doing it myself just to cut down on outside noise.
I used e-dead in my previous vehicle (too lazy to do it in my 4Runner yet). It's much cheaper than Dynamat and a very good product. Look at Elemental Designs they have a lot of different varieties. I used the liquid stuff then the aluminized stuff. Makes for a very quiet ride. The liquid stuff paints on very easily and basically turns into concrete. If you go that way I'd suggest getting both colors. That way you can see this layers have been covered properly, alternating each layer.