If you want to stop the racket of a diesel, or as much as possible, you'll have to use mass loaded vinyl, MLV. Dynamat just isn't enough. It's good for giving good doors a great feel and helping keep your bass in the truck, but that's about it.
I've sound -deadened every vehicle I have ever owned since highschool. I've gone as far as doing 95% of the vehicle interior, excluding certain small pieces of the firewall. The results are diminishing returns like anything else.
My current truck has two layers of Dynamat, one layer of closed cell foam, and one layer of MLV, everywhere except the firewall and roof. Would I do it again, idk. Can set my cruise at 85mph and still listen to my music at a moderately low volume, yes. Can I hear my terra grapplers hum at 65mph, not really.
It has quieted the interior enough that I can hear my stock vgt sing at 1000rpm. But it's a lot of work if you want it done right.