It all depends. Yes you will need to replace the seal, but if dirt got up there, the yoke may have gotten scored. Another common reason for a leak is that you lifted the rig and didn't lengthen the drive shaft. That pulls the yoke out far enough that it wobbles. This can wear out the output shaft, the yoke, the slip bearing as well as killing the seal.
The only way to tell is to get under there and pull the shaft and start looking.