The upstream O2 sensor is usually located in the exhaust manifold, close to the engine. Its exact location will depend on the specific engine configuration, but it's generally on the passenger side (right side) of the engine, near the exhaust manifold.
The downstream O2 sensor is located in the exhaust pipe after the catalytic converter. Finding this one is more challenging as it's further back in the exhaust system. It will be closer to the catalytic converter on the passenger side (right side).
To be absolutely certain, you should consult a repair manual specific to your 2000 Ford Ranger 3.0L engine. A picture diagram in the repair manual will show you the exact location. You can find these manuals online or at auto parts stores.
