If you're a gambler then don't replace the water pump. If you're planning on keeping the truck another 100k, it seems like a little investment now will protect you later down the road.
Also, while the pump is coming out and the belt, make DANG sure you have the belt tensor replaced to. If you don't You will be replacing it also, down the road. When the belt tensor goes to will sound like a horrible rod knock.
I've seen way to many posts like this one:
http://www.4x4wire.com/forums/showfl...=5&o=1&fpart=1
Do what you want pay the money now while it is all opened up, or risk redoing the work over again down the road.
-Ryan