A nice clean sound deaden job is a must for a tailgate install, it cleans up the inside, helps isolate sound resonance, but the best part is that the shiny backed stuff really brightens up the black hole and helps you find your stuff when you need it.
I’ve posted info on the different materials out there in forum posts and threads, but I’ll re-post some of it here for consolidation.

So, the first question I usually get is “how much do I need?”
It seems to depend on how good of a job you do.
For a 60 gate, 6.75sq’ will cover but 10sq’ would be safe.
The 80 needs about 7.5sq’ but 12sq’ would be safe.
And the 100 Gates are safe at 12sq’
Give or take on all of them depending on how nice of a job you do.
You’ll have to get your own ruler out for the side panels since the amount used there will depend A LOT on the installer.

Depending on the condition of your sheet metal, you may want to spend less or more on the sound deadener.
If you want the best cheap option this works great:

You can also order it from Aubuchon online here
And most Lowes home improvement centers carry handy 6″ rolls of it now.

I keep some rolls of the 6″ wide stuff in my stash, I can set you up with some and send it along with your tailgate, you can get it for around $16 from Lowes, $20 from me (offsets the added shipping weight).
The stuff is good, sticks great, and works. But it is asphalt based and not very technically advanced. I have it because I can’t justify spending 5 times as much on something to stick to rusty metal.

When I am doing higher quality work I tend to use RaamMatt:
It is the best, most cost effective, balance that I have found. No smell, easy to work with, sticks great and has high temp resistance. (in my experience)

When comparing prices for your choice it’s not exactly apples to apples, you need to take the thickness into account.
The peel-n-seal (which works great and I use it for almost everything) is 45mil and is $1.42sq’
If you break down the sq’ price of the RaamMatt as if it were 45mil (since it is actually 60mil) it would come out to $2.80sq’

Those are the two items that I use.

The thing to remember is that there are a ton of opinions about this stuff, and folks seem to get very adamant that their opinion is the correct one, so take it all with a grain of salt.

The only thing that I really don’t like to see is people waste their money on Dynamat, it’s a great material, but no better than any others and for comparison you end up paying $14 – $19 sq’ for nothing other than the name.


A note on actual “sound deadening”:
In the info above I am not talking about actually trying to stop sound, I am talking about lining the new interior storage space with a bright clean material.
If you want to get into the science of what actually needs to happen to deaden sound this is a great source of info:
That is a great explanation of what is the best and why.

I see a lot of folks go through the effort to cover 100% of their trucks with these foil backed damper materials in order to “sound deaden”, but that is not really what it does- It adds mass and dampens noise by virtue of damping vibrations or resonance. If you can tap on the place you want to use it and it reverberates, good, perfect place for it. If it does not reverberate, that material will do exactly nothing for stoping sound, it is not a sound barrier.

To actually stop sound, you need things like carpet and foam that actually kill noise, they break up sound waves, shoot them off in other directions so that something else can break them up more and eventually dissipate them.

If you want sound to actually die, you need soft, open material.

The aluminum backed material is great for sealing off areas though, I use it to cover the inside panels of doors (after putting some squares of in on the outer door skin to stop reverberation) and the openings in the door pillars of the body, etc.

The products I mentioned above are the ones that I choose to use, they are cost effective and work well.
For the best materials I have found and used with the best results (and the most money spent) the product line from here:
Covers a great range of applications and actually will help you kill noise rather than sell you a bunch of sticky foil.
(RaamAudio also has a great self adhesive foam mat that I use a lot of)

Read through the sound deadener showdown link above to get an idea of what needs to happen and then take a look at the second skin line.
Good luck.