Whatever you end up doing, you're going to have to pull off the drive shaft. You don't say how many miles are on the truck, but you might consider replacing the U-joints while you're at it.
As for the name of the part you're talking about, its on the tip of my tongue but won't come out my fingers.
Go see your handy parts guy either at a parts store or Dodge dealer and they can bring up a complete diagram and you can point out what you feel the guilty party is.
Any leaks either on the differential or transfer case?