Can attest to the horrid drop-rate. Only recently rejoined the game and jumped back into goldsmithing. Spent the better part of 2 weeks in July at roughly 4 hours per day to no avail. Decided to randomly try again today and used Blm to cook everything en'masse. Got it in less than 30 minutes =/
Drop rate is excruciatingly low on this. With Full CD prowess and 500+ chests opened, I still haven't been able to find one. Wasted 20+ hours locked into the zone over 2 days with nada to show for it.
Finally got mine, yes it comes from brown casket in Den of Rancor.
I made my own thief's tools as I went to conserve inventory space. 8x stacks of copper ingots, iron ingots, and yew lumber, and it was my VERY LAST THIEF'S TOOL that yielded a goldsmith's torque.
Took me about a week playing 4 hours+ daily, opened about 150-200 boxes.