Normally, you will get a "Check Engine" light if your HO2 sensor or A/F ratio sensor are out of spec. Sounds like your truck is running fine anyway.
A/F ratio sensors are much more expensive than $50. Find out what sensor you have (and a part number for reference) with your VIN at a Toyota parts department before you go off madly buying sensors. Some have HO2 sensors and some have A/F ratio sensors.
http://www.autoshop101.com/forms/h37.pdf