Where is the NOx sensor located on a 1987 Chevy truck?

A 1987 Chevy truck did not have a NOx sensor. NOx sensors were not commonly used in vehicles until much later, typically in the mid-to-late 1990s and beyond, with the introduction of more stringent emission regulations and technologies like Selective Catalytic Reduction (SCR). Your 1987 truck likely used simpler emission control systems that didn't require this type of sensor.