Where is the downstream oxygen sensor on a 91 ford ranger 2.3 liter engine?

The 1991 Ford Ranger 2.3L engine does not have a downstream oxygen sensor. Downstream O2 sensors were not commonly used in vehicles until later in the 1990s as a part of more sophisticated emissions control systems. Your Ranger only has one oxygen sensor, and it's located in the exhaust manifold, upstream of the catalytic converter.