I thought most any car, 2wd or 4wd, would have to be in nuetral to flat tow, ie; transmission & transfer case in nuetral. My guess is the only Nitro that can be flat towed is the SXT with the manual trans.
I had a Toyota 4'Runner with an auto trans years ago & it had a cooling problem when being towed with all the tires on the ground. I guess the trans fluid wasn't being pumped through the tranny if it wasn't running.
Edit: I assumed that Robin meant towing with all 4 wheels on the ground. Maybe you can clarify exactly what you mean by flat towing.