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I don't know of an o2 relay.
Turn the key 1 click, not to run position, pull it to neutral and start the truck and take it to have stoplight switch and/or the shift lock actuator tested.
Check the battery connections, possible jump may prove it.
Verify that it's not running lean on that cylinder.
Parts fail, what can you do but replace it.
Using a fuel line type wrench works better to loosen the nut, then you can use a regular wrench to remove.
Check the shifter cable clip at the transmission end.
Test blower, blower resistor, and/or switch.
On the left of this post is a search window, put emission systems or evap in the window and click GO. This will take you to an area that will give you more information.
Use the Estimator on top of this page for your zip code.