You may have never seen this on your truck (or maybe just the part that left over after they break). It's a rubber mount that screws into the passenger side rear of the valve cover that the air filter housing mounts to. If your air filter housing feels loose or rattles around, it's a good chance your rear mount is missing or broken. When it is, it puts excess stress on the other mounting points, causing them to fail earlier.
** hey! I agree these are kind of pricey. It is a OE toyota part. I've tried finding them on the aftermarket (including the ones for Mercedes, Volvo and BMW), but I've yet to find one that fits correct.