Like Michael suggests, look at the easy stuff first- make sure your tires are aired up, replace the air filter. 14-16mpg is pretty typical for a stock 1998+ 3.2 4x4 w/ AT, that's not doing straight hywy driving. One of your pre cat O2 sensors may be on it's way out, but not totally failed, as to set of a CEL. If you have over 100Kmi, it's not a bad idea to replace those two O2 sensors. Any major malfunctions with this truck will light up the CEL. G/luck
Joel