Are you sure the clutch is working properly? When the engine is first started (cold), the clutch should drive the fan for at least the first few seconds -- and with the hood open, and the engine at fast idle due to the cold start, it will be VERY noticeable -- both noise and airflow.

After the first few seconds, the clutch should release and the fan should almost freewheel, the noise (and airflow) should be much less. Depending on the ambient air temperature, with the engine at idle, the fan may not have to "kick in" to draw enough air to keep the engine cool. However, at the slow speeds you describe, and under load of moving the vehicle on the trails, the freewheeling fan (if the clutch is not "kicking in") may not be pulling enough air.

To check this, try to introduce a restriction (i.e. a towel, cardboard, etc.) in front of the radiator to block the airflow. Start the engine (preferably cold, so you can get a feel for how much air and noise the fan should make when the clutch is engaged), and then let the engine idle. If the clutch is working properly, the fan will disengage after the first few seconds (and quiet down), and then (time depends on the ambient temp, etc.) after a while, when the small amount of air that is coming past your restriction gets hot enough, the fan clutch should sense the heat and engage again, making the same amount of noise (and pushing the same amount of air) as it did immediately after start up.

There is an "old wives tale" that says that fan clutches lose about 10% of their efficiency each year, or 10,000 miles. I've found this to be almost spot on with the GM vehicles I've owned, and on the other hand, have had a Toyota 3.0 go well over 100,000 without needing a new clutch.

If the radiator is not partially (or mostly) clogged, and the thermostat is in fact working, my money is on the fan clutch as the culprit. Good luck!