I have 20 inch wheels on my 2500HD for years now. There are plenty of OffRoad tire choices, KO,KM,STT, MT/R, on and on. I usually run a 285/65/20.
While I am not a big fan of airing down,I have never had an air-down issue, granted I do not run a 6psi, but I can certainly drop them down some...