Not sure how many on your truck but on most vehicles there is one at the collector of each exhaust manifold (these are "upstream" sensors) and there is usually one after each catalytic converter, or on each catalytic converter (these are "downstream" sensors). The upstream sensors are really the important ones that give feedback to the pcm for air/fuel ratios. The downstream ones are primarily just to monitor catalytic efficiency.
Do you have a CEL?
Some vehicles around your truck's age I think had 1 upstream and 1 downstream sensor, or maybe 2 upstream and 1 downstream sensor.
My 2002 had two. One located prior to the cat, and the other locate after the cat. Just look at the exhaust pipe and you will see a protrusion about 2 inches long with 4 wires coming out of the top of it.