99 GMC Jimmy SLT HEADACHE

Sounds typical of the injectors. While you could have a bad one, it is more likely they are mismatched. Chrysler is one manufacturer that has almost no trouble with injectors because they buy them from Bosch in matched sets. GM just grabs a handful out of the box and throws them in. Because they flow at different rates, you will always have a few cylinders running too lean and a few running too rich. That very often results in the random cylinder misfire codes that GM is famous for. Since you already made the dandy observation that one cylinder isn't contributing his fair share, you might consider swapping that injector with one of the other ones to see if the problem follows the injector to the new cylinder or if the same cylinder still has the problem. When one injector doesn't squirt the proper amount of fuel, the normal amount of air is still going through that cylinder. That unburned oxygen is detected by the oxygen sensor as a lean condition. The Engine Computer will request more fuel from either all six cylinders or from all three on that side of the engine, depending on the model and year. No matter how much extra fuel is added, there will still be that unburned oxygen. O2 sensors don't detect unburned fuel, just oxygen. Fuel mileage will be terrible too from all the extra fuel going in.

There is a place in Indianapolis that rebuilds injectors and sells them in matched sets. The fellow's name is Jim Linder; not sure of the name of the company. They buy used injectors too to keep up with demand. Many GM owners comment that their engines never ran so smoothly until they replaced the injectors.