Doesn't seem to drop Goblin Slaver's Whip. Killed him many, many times. Listed as a rare drop, which should be somewhere around 5-10% chance. Statistically very unlikely you wouldn't see one in 50-100 Grachnists.
Also have never have seen one for sale on the server or wielded which seems unlikely for a commonly farmed NPC for the Shrunken Goblin Skull Earring
Post from Allakhazam, seems it dropped from Feb 2001 and on (possibly earlier?):
http://everquest.allakhazam.com/db/item.html?item=208