I am saving for the Toyo AT's. Supposedly they are great for the heavier trucks. At least you don't have the cummins which adds quite a bit of weight to the truck. I had the cooper STMaxx on my Tundra and they were a great looking, sounding and wearing tire. I have the Cooper ATP (which look like zeons) on my 4runner now. Not a bad AT tire and really quiet.
These 20" rims leave little options, especially in factory size. Going up a few sizes you have more options but man are they pricey. On average they are $350 a tire. I would avoid BFG KOs as they wear incredibly quick on HD trucks I hear. I decided to just wait until the OEM wear out which will likely be within a year at my current rate.
For good information on the rams, check out Cumminsforum.com. I know you have the 6.4, but the rest of the truck is identical and there's a ton of good information and knowledgeable people on that site. Lot of 6.4 people there too.