FWIW, I sound deadened my cab, it did make a pretty good difference. what I did was get a couple rolls or Raamat and used it in the most needed places (i.e. I wend around and tapped on the sheetmetal to find places that resonated more than others, like big flat areas, etc) the 2 rolls was just enough to cover all the places I felt were really necissary, for me I used a tennis ball to help smooth it and push it on to get good adhesion (I also cleaned all the surfaces beforehand with denatured alcohol, to promote proper adhesion). then I had a bunch or Second Skin Spetrum (the paint on stuff. it actually is pretty good stuff), I think I had around 4 or 5 gallons of it, and after the raamat was on really well, I went through and put a few coats on everything, letting it dry for at least 12-14 hours before the next coat. Then after the last coat was dry, I used some spray glue and glued some ensolite closed cell foam and glued it over the major areas (i.e. floor, inside doors, etc) and did double layers with the extra I had in spots like under the sub (its a down firing single 10") and behind the door speakers. after that, I got some expanding foam from Home Depot, its called 'Great Stuff' its expanding insulation to fill the gaps between window frames, and used it to fill in and add ridgitity and insulation inside the gaps between the rear pillar sheet metal (this was after running my rear speaker wiring too).
thats the majority of what I did in the cab, but you'll want to focus alot on the inside of the doors. I did same Raammat first, Spectrum layers second, ensolite foam third, on every surface of the door I could get to, this includes the inside of the door too. then I sealed the major openings in the sheetmetal with sheetmetal (which had also been treated with raammat and spectrum) and very small self tapping screws, which will let me remove them and replace them should I need to for repairs or something in the door. I focused alot around the door speaker, I did double raammat and triple ensolite behind the speaker itself, and I also made custom baffles out of 1/2" thick corian scraps (the countertop stuff), this added alot of ridgitity to the speaker mount which is sorely needed. and I used a little plumber's putty between the baffles and the door sheet metal, and the speaker and the baffle, as well as between the speakers and the sheet metal on the rear speakers, which helps seal any cracks or small gaps. some people use MDF for building speaker baffles, but the problem with that is if the MDF isn't properly prepared and it gets wet (its in the doors), it will just fall apart. I used corian because A) its completely unaffected by water, B) its solid which provides good ridgitity, but its also more dense than like MDF, so you can use thinner pieces to achieve the same or better results ( I used 1/2" corian, most people talk about using 3/4" MDF, which may cause clearance issues with the door covering panel back on), C) its fairly easy to work with using wood working tools, D) it can be somewhat inexpensive if you can find some free or cheap sample pieces, or leftover scraps from someone (or a buisnesss) who recently instlaled countertops. those foam speaker cups will do more harm than good, they choke the airspace needed for the speaker driver.
thats about all I did for the soundproofing. it did really well, there is still some wind noise from the weather stripping on the doors, and from the windows that I haven't tackled yet, and when I installed my T56 trans, I had to cut a hole in the floor, which lets some noise in, and I'm going to work on sound proofing that better in the future as well. but over all it does well, it not only keeps road noise down in the cab, but I can listen to my stereo without bothering people around me. I did all this as a 'budget' sound proofing, which it kinda was, but I kinda overdo things too, so not everyone will need to do everything I did to get good results.
I also wanted to use the stock speaker covering, etc, so eveything would look like it was stock, but when you turn the stereo on you get a pleasant suprise. with a few modifications I put the component tweeters mounted in the stock location (but angled slightly for better aiming), and behind the stock tweeter screens, the 6 1/2" speakers in the doors (even with my baffles) fit under the stock covers, I didn't do alot to the rear speakers as far as baffling and stuff goes, since they are a minor part of my system, but they also are in the factory location. and my sub fits under the rear drivers side of the extrended cab bench and is covered with simmilar grey carpeting, so you really have to be looking for it to notice that it's there.
and its done really well over the past 2 years or so.
also, not for soundproofing, but for mounting my amps, while I was in doing everything I glued some small slats of MDF to the back wall of the cab (in between the folds of the sheet metal), and then when I was ready to install my amps, I just used regular wood screws to affix my amps to the wooden slats, so I wouldn't have to screw through the cab sheetmetal.