Not an expert, but sounds like it could be your differential gears.
The Jeep grand Cherokee's of the 90's and early 2000's had this same problem, had to do with different metal alloys expanding and contracting according to their different physical characteristics, created wear patterns that made noise. The metal shavings support this in your truck.
There have been a few folks with Tacos who have had problems like yours. Keep searching. Might have to wade through some fecal material, but the Tacomaworld guys have addressed this question a bit...again, you'll have to get knee deep in a bunch of offended 12 year olds attempting to swear their way into respectable pirate-hood, but you may find a few pearls amidst the refuse.