I just did this quest (along with the curse collector quest). It was easy after a little waiting around for rain (which wore off right when i got to the stacks >.< lol). I'm thinking the ??? could spawn anywhere around that little island the stacks are on.
If it helps at all, here's a pic of the map where I found mine:
http://img43.imageshack.us/img43/4958/screenshot00060zt.png