I honestly do not know if the serp belt can be changed with the fan shroud and fan in place. If so it's going to be a really tight fit getting to the serp belt tensioner and if the tensioner is bad (I hope not as it's pricey) I'm sure you will need to remove the fan and fan shroud.
In the event fan and fan shroud removal are necessary you might check out my comments at
viewtopic.php?f=5&t=66086 . The fan nut is 36mm and you can see what I mean about the allen wrench at
http://www.beesvillebeefarm.com/jeep.html just scroll down a couple of images past the picture of using a palm nailer to jar the nut loose. Bottom line is I'd strongly recommend one of the various fan shroud mods for future working ease, you will know what I mean after you pull the stupid thing, if you need to pull the shroud and fan.