I work for an off-highway equipment manufacturer in their sound group. Most of our sound absorbing/dampening products come from Soundcoat
http://www.soundcoat.com. Their products are not cheap, but this is exactly what the type of application for which they are used. They can probably even make some recommendations over the phone.
I haven't driven a deuce, but from my reading here, most of the noise is low frequency exhaust and engine noise with some turbo whistle thrown in for fun. Low frequency noise is not absorbed well, you must block the noise from coming into the cab in the first place. I don't think you can describe these cabs as "well sealed". So good luck with that. The sleeping mats some people mention are closed cell foam and will not work as good as open cell foam (not to say they won't help some) to absorb sound at higher frequencies.
Just avoid the melamine foams because they are very fragile.