Does it overheat on the highway? Around town?
If you have a bad radiator, you may need a new one, but you don't need a 'performance radiator'. The factory radiator has more than enough capacity to cool the 3.4 in any weather we're likely to encounter in the US. As has been noted above, and in many other posts, many of us have wheeled in very hot weather and humidity with no overheating (I sometimes even run the A/C with the windows downs while wheeling when it's really hot). The cooling system on these trucks is very good.
So, if you have a problem, there are 4 possibilities.
First, you could have a clogged (or damaged) radiator. If it is clogged not enough coolant will flow through it and the motor will overheat. If that is the case you will likely have overheating on the highway and driving around town too. There probably is just as much load on the motor getting up to speed on a freeway ramp (though for shorter periods) and going stoplight to stoplight as there is off-roading. Your radiator could also be very dirty, clogging the fins so not enough air can get through as well. But, if you don't overheat in other conditions, it's likely the radiator is not the problem.
The heater core doesn't contribute anything appreciable to cooling, except when the heater temp is all the way up and the fan is on high (and nobody drives like that in the summer to keep their car cool - that would be a design flaw).
Second, you could have a bad thermostat. It may not be opening fully and under high loads not allowing enough coolant through. This is possible, but not likely, and again, you would probably see overheating in other conditions.
Third, it could be a water pump, but that is very unlikely. These pumps either leak or the bearing goes bad (though I haven't heard of that). The design makes it virtually impossible for one to lose efficiency and not pump enough coolant.
Fourth, it could be the cooling fan (or a broken shroud). If it is not overheating on the highway it is because there is enough airflow through the radiator to remove the heat from the coolant. When you are off-roading I'm assuming you are moving very slowly and therefore not a lot of air is passing through the radiator due to the truck's speed. That's when the cooling fan works - it must pull enough air through the radiator to remove the heat from the coolant. If the fan can't do that, it will overheat at slow speeds but cool fine on the highway, or even around town (though it might creep up a bit in very slow traffic or at long stoplights).
That's about all that can go wrong, other than things like a bad head gasket (but you'd be losing coolant or have other symptoms) or a bad radiator cap (probably wouldn't overheat but you'd be losing coolant or have steam coming out under the hood).
If you suspect the radiator is clogged, I'd remove it and flush it with a water hose. If it flows freely it's not bad. If it is, get a new OEM radiator; if not, put it back in and check the other things.
