Well then, you might want to test the O2 sensor per the FSM procedure and/or install an air/fuel meter to keep an eye on the mixture as you drive to see if that sheds any light on the problem cause:
- http://www.4crawler.com/4x4/CheapTricks/Gauges/index.shtml#Air/FuelRatioGauge

Injectors can also be clogged:
- http://www.4crawler.com/4x4/CheapTricks/EngineMods/index.shtml#Injectors