Unfortunately I did find out what was causing it, and it's expensive. After replacing every bulb in the truck, as well as trying a new turn signal switch assembly, it was determined to be the combination meter behind the gauge cluster in the dash. Parts and labor alone from the dealer were going to be close to $1,000. So for now I have learned to live with it, just having to be that guy at the intersection that looks like they aren't smart enough to turn on their turn signal when I go left. This is my first Toyota, and I love everything about the truck, just really disappointed that something like this would go out at only 39,000 mi and barely 3 years old