2 valves on regulator.
OK, this truck is equipped with Three Way Catalyst system (TWC).
With TWC package, there is a fuel injector beneath the main mixer body. The injector has a separate fuel hose leading to the regulator.
You need to check the wiring AT THE INJECTOR plug-in connector to make sure one of the 2 wires has not broken, rendering the injector inoperable.
The injector is only a "make up" or "fill in" fuel device which adjusts the air/fuel ratio according to info the TWC controller (ECM) gets from the O2 sensor. Loss of injector function can cause stumbling coming off of idle and also a loss of net power under heavy engine load conditions.
If you find a broken wire at the injector connector, a replacement connector service pigtail kit is available from Toyota, part number 90957-U9012-71.
A clogged or otherwise defective injector can cause the same problems as a defect in the injector wiring.

On the subject of TWC, make sure that the "check engine" light is working on the monitor. To test that, look in the upper right corner of the monitor panel to see if a red light with an engine icon comes on when the key switch is turned to the ON position, engine not running. The light should burn at that time and go out (if all is well) after the engine starts.
If the light is not working at all, the bulb has probably failed.
If the light works and stays lit after the engine runs, it means the TWC controller has turned the light on in response to a defective condition somewhere in the TWC related conponents.
TWC related components are O2 sensor, MAP sensor, fuel injector, and the TWC controller itself.
Wire breakage is all too common on these trucks and Toyota has made all the usual connectors available as replacement parts items.
The 4Y engine is such a shaking/vibrating beast that it is not unusual for wires to chafe or break inside the engine loom work where the harness jumps over the valve cover area.