Did you ever figure it out? I'm no mechanic, but I'd assume it's related to the fuel management. Maybe your injectors or fuel pump... Does your truck ever run funny? Which end of your Tacoma do you smell it the most? Is it possible that your engine isn't igniting the fuel efficiently and you're smelling whatever isn't burning off? Maybe one of your fuel lines crapped out under the hood. Like I said, I'm no mechanic. Just thinking of possibilities...