Fred's right. There are a few different products available. I used Dynamat on an 85' Mustang years ago when it first came out and I was pleased as could be with it. In addition to making a very noticable reduction in road & engine noise the radio sounded much better. I took a friends recommendation and put a double layer on the doors and the added weight along with the sound deadening made it feel like I was closing the door on a Mark VII instead of a Mustang, a heavy feel to the door & a solid thunk when it latched, not that hollow sound they usually make. REAL sweet. Only downside to the deal was the cost. I seem to remember it running me close to $750 to do the *entire* car, floors, sides roof & double layer on the doors. It wasn't cheap but it really did sound like a lincoln when I was done & I never regretted it.
Nowadays you can get knock-off's that work & sound just as good for a fraction of the price. I've used the Brand-X stuff on a couple of my 78's and been very pleased with both the cost & performance. I've never really insulated another vehicle as well as I did the Mustang but with my trucks I kinda like hearing the Ground Hawgs sing, but I don't like everyone in the truck having to shout to be heard.
You can take a couple of approaches with it. The rubberized undercoating is effective to a degree and helps a lot with rattles but it has a real tendency to rub off. You can use the Dynamat or a generic knock-off od it and that's very effective and you can also use a combination of the two. I've sprayed a heavy leyer or undercoating all thru the truck and then covered it with Dynamat and really gotten excellent results. When using both products it's best to only do it in places where you're able to either lay carpeting over the dynamat or cover it with a trim panel like a door panel or the rear side panels because otherwise the dynamat can/will shift and just peel the undercoating right off.
However you choos to apply it & in whatever combination, the sound deadening mats are pretty good products and well worth the $$$ and the time/effort to install. Make no mistake, it's a bear of a job but if you're looking to remove all your trim *anyways* and you're maybe kicking around a new carpet then sure, I'd really suggest doing it because when you're done you'll be more than pleased with the results. You might consider going with the double application for the doors (and maybe tailgate since it's such a big resonating chamber) not only because they generate so much noise by being big hollow airspaces but when you double up on it (especially with a layer of undercoating on the metal first & then another coat between the layers of matting. Sort of a double/double application) the door gets heavier & *feels* a LOT nicer when it closes. I found that I really liked the feel & sound of the doors closing as well as the benefit of all that sound-deadening material, it makes the truck a LOT quieter and it really does bring you up & into Lincoln territory as far as road noise & issues like that. I think you'll be happy if you go that route. I've yet to meet anyone who did it and was unhappy with the results.
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