On my 3126-E I believe it would be a tough repair because of the rear radiator. And I know I could not get to it from the box under the bed.
I believe what I would do is remove some of the parts that are blocking you from getting to the fan. I would make some room to be able to work. I would probably remove the big CAC hose on the passenger side and a few other things to get to it. If you come in through the drivers side I would remove the muffler and air filter. It will not be easy, but it can be done. I do not believe you need to remove the radiator. Not sure!