The air injection reaction system (AIR) is on all 2004+ 4.2's. There is an electric blower under the truck, tucked inside the frame rail about under the drivers seat. The blower draws it's clean air from a hose tied into the air inlet tube between the airfilter box and throttle body, pressurizes it slightly and blows this clean air into a port in the cylinder head directly above the exhaust manifold, dead center. There is a can shaped check valve bolted to this port with the air hose clamped to it and an electrical connection. The most common failure point of this system is this check valve. Remove the hose, the electrical connection and 2 bolts and it's off. Very easy. Problem is, it's about a $125 dealer only item.
The check valve gets corroded/cruded up and doesnt seat. Let it go long enough and moisture from the exhaust can back flow into the system and destroy the $$$ electric air blower. I'd start by removing and checking the valve and check the hoses for water.
G/luck
Joel