Depends on what you want to do with it? Most of the mods you can do to the 5.9 are going to sacrifice low and power and torque for the high end... not really great for towing were you want your power from idle - 3,500 RPM or so.
I put in intake and new muffler on mine and it breathes a little better on the high end. I have a set of 4.10's sitting next to me to bump my RPMs on the highway a little. 5th is a little too tall with the Jeep behind it.