Yep AC idler. You want to order a pulley from a Tacoma rather than the 4Runner. The Tacoma pulley is beefier and the bearing is replaceable.
The factory one is #8844035060 and is $41 on Camelback.
The one you want is #8844004040 and it's $31. So it's beefier, serviceable and cheaper. Not sure of any reason to get the factory one, unless you're going for lightweight, the factory one is 9.7 oz and the Tacoma one is 13.1 oz. I'll do a write up one of these days as I have side by side comparison pics.