Where is the ECU located in the 2008 Tundra?

I'm assuming You want the primary electronic control unit (ECU), vs any of the various modules (like the one in the engine near the air filter) that talk on the CAN network in Your 2008 Toyota Tundra. So, that's going to be under the passenger side dashboard, behind and under the glove box; it's a flat box protruding from the firewall, surrounded by a black rubber gasket, and a large bunch of wires going to it. A new ECU for Your vehicle's going to run almost $500; remanufactured ones can run about half that, with AutoZonoe selling part ECC11316 for $255. If You're not familiar with what these look like, check that out, at least, before You start poking around under the dash so You'll have an idea of what You're going to replace.