Just some FYI for those not immersed in the world of audio, but want to be. "Deadening" materials serve 3 different purposes based on the product (in general). There's products that mass-load (like vinyl, lead, etc.), products that create damping via a constraining layer, and products that decouple. Old-timey methods to deaden usually involve adding mass, which reduces the resonant frequency and lowered the efficiency of the transfer of energy from vibration into sound. That's Dynamat's original claim to fame. This gave way to Constrained Layer Damping, which is a form of absorption through the mechanics of friction, is a great way to reduce resonance. The science of it is actually fascinating. In simple terms a visco-elastic material is combined with a constrained layer (like aluminum), and energy in the form of vibration is converted into (low level) heat. The amount of energy taken up by this is pretty amazing, and it is very effective. The last type of product is a decoupler. This is usually foam, but could be batting materials like jute or sometimes more exotic products like 3M Thinsulate. Typically, making a plan that involves all of these types produces the best results. For instance, installing CLD, then foam, and then MLV (Mass Loaded Vinyl) works especially well together, and is considered the basic "process" for doing high quality deadening work.
I'm a DIYMA member and in the forum, we have a guy named Nick that's started up his own sound deadening business, called Resonix. Nick has researched this topic a great deal, and has found some killer products out there that do what we (in the audio hobby/industry) need so badly, such as a closed-cell foam with the perfect properties to decouple without adding a squeak or making a panel hard to put back on. I plan on using quite a bit of that in my FJ when it is time to do the audio overhaul I have planned in my head. I've used lots of different deadening materials in the past, and the one I've kept returning to is Knu Knoconceptz Kolossus, at least for CLD (Constrained Layer Damper). Prior to Resonix showing up as an option, I always landed on Knu Kolossus and used it with great results.
One tell-tale of good mat vs. not-so-good mat is how it is packaged. If it comes on a roll, it typically will not be the product you want to put in your vehicle, and instead maybe consider that for your house's roof. Good quality CLD is going to come flat-pack in rectangles/squares. It is better on the product to be packed like this. I also like using foil-foam-foil insulation products sometimes. These are usually mid to high-grade insulation products for things like trailers, garages, attics, and so on. I use a product called Low-E that has worked well in applications where I can't add MLV, due to weight concerns. Products like this don't weight much at all (they are mostly air and aluminum!), but it is effective reflecting and absorbing heat as well as some sound. I found this quite useful building sports car setups, where sound is important but adding hundreds of pounds would be unacceptable.
For my FJ, I'm not sure what I'll end up using but I'll probably start with the Resonix CLD and foam, and see if MLV is even necessary.
edit: It is important to mention that studies have shown CLD coverage only needs to be in the 20-30% range (typically only on the flat surfaces) for it to be effective. Deadening a curve does nothing, because the metal is not weak and resonant there. So to line the whole vehicle in CLD is a waste of materials. Now, lining in MLV or decoupling foam (which needs to be closed-cell to prevent moisture retention) should definitely be considered a full-coverage thing if you can get it. But if you are after bang for the buck and bang for the pound of CLD material, heed the ~25% range of coverage target and don't worry about blanketing an area.