It "may" be the engine fan but it might quite likely be the exhaust. A number of owners have noticed this whenever they have a turned down exhaust pipe extension that aims down. By loosening the clamp and rotating it so that it aims more to the rear and less down it's been corrected. If your '08 is a Freightliner with the square diffuser you may be limited in your ability to do this though.
If it is indeed the radiator fan it's possible that your mudflap/splash shield is hanging too low and kicking up dirt so that the fan can catch it and spit it out through the radiator. It will pick up air from below and spit it out the radiator so if you are stirring it up that would contribute.
I also try not to kick the back end out and drift through the corners with opposite lock on dirt roads.