When I replaced my heater core, I removed the entire interior(dash, supports, heater vents etc.) and lined the entire firewall w/ extreme dynamat. This cut down engine noise quite a bit.

I also did my doors w/ the stuff. After installation, the doors had a much more solid feel to them when you shut them and gave them less of tin can feeling. But unfortunately while on the trail my truck fell into a tree thus smashing in my door and I have since replaced it -- luckily it was the passenger door and I don't notice the excess noise as much.

Another thing I did that cut down on road noise considerably was herculine the interior of my cab.

But don't get me wrong, even after all of those things, it's still pretty damn loud going down the highway w/ 33" mud terrains. It just made it A LOT more tollerable.