while you have the front of the engine apart you may want to pay close attention to the timing belt and sprockets too, especially the oil pump sprocket, they have a tendency to wear out after a period of time. If the truck runs in a dirty environment that is :o)
if the belt has ANY abrasions or cracks in it, replace it. If the water pump is showing ANY signs of leaking? replace it.

oh yeah, when you put it back together and have to set the timing? MAKE SURE you time in the oil pump sprocket too, yes there is a timing mark on it as well and if it is not timed along with the rest of the engine you will get a vibration when the engine is running.

OF COURSE if your just going to pull off the pto shaft and pulleys and just pop out the seal and stick another one in there then forget all i mentioned above. I just mentioned all that since you are going to be taking it apart ;o)

ummm....oh yeah again, is this thing leaking oil out anywhere else? valve cover perhaps? if so you might want to check the pcv valve in the valve cover and make sure its free and clean. If this engine has alot of hours on it , it might just be blowby creating high crankcase pressure and replacing that seal will not help for long, it will just come out somewhere else ;o)


hope this helps somewhat ;o)