The bubblewrap stuff is OK, but for real sound deadening I was looking at installing either B-Quiet or Cascade products:
http://www.b-quiet.com/compare.html
http://www.cascadeaudio.com/products.html
I think I'll purchase the Cascade products because they are vinyl based, not asphalt based, and from what I understand the asphalt products can give off a smell over time.
I plan to make the truck pretty quiet (for a D90) - doors, firewall, floors and (eventually, when I purchase one) fiberglass top will all get the treatment.
I've seen some kits in one of the Land Rover monthies which had firewall and engine insulation kits also -- I might look up those while I'm at it.
The guys at Expedition Exchange soundproofed their D90 and from what they say it's darn quiet at highway speeds.
Look, it *IS* my daily driver, after all (for those who think the only good D90 is one buried up to the hood in muddy water
)