Hey Tacomaniacs! Stumbled across this post while researching my own sound deadening project (for a Honda car), and wanted to thank ItalianStylion and weigh in as well. My advice is - USE MLV!
I had reached the point in my install where I would either simply re-install the carpet over the Second Skin constrained layer damper and closed cell foam, or add a layer of MLV.
Like some of you, I debated the use of MLV (more money, not sure of the benefits, hard to source, etc.). I decided to go all out. I found a good deal on MLV at Super Soundproofing http://www.soundproofing.org/
. I installed the MLV on just the floors about a week ago, and the difference was *very* noticeable vs. CLD and 1/4" foam only. I have yet to do the doors, but I expect even more improvement from that step.
My advice is that if you are going to go to all the misery of ripping your interior out, just do it right. The MLV is well worth the effort - unless you're some kind of hypermiler, but then you wouldn't want those heavy subs and amps, either.