I saw 1-2 MPG increase when I replaced upstream o2 sensor. 19ish city driving

99, 5sp, 4wd, 265/75/16, heavy load in back, 130k miles

5-speed with 4:10 gears. Auto hurts mileage.

-Charlie

Yep - the overdrive ratio in the auto is better but the auto still gets worse mileage.

Nonsense, you can get more than 16. Key is to keep the rpm's under 2k. Don't drive faster than 55 mph. Learn to control your acceleration. NEVER accelerate uphill unless it is an emergency. NEVER use drive through's or do any idling unless it is a traffic light. These items alone take me from 17 to 20.

Some people think it's crazy, but I've achieved 28 mpg in a 2wd 2nd gen Tacoma. That was with a 3" lift. Based on my experience with the 3rd gen 4runners, 18 mpg with some armor is totally realistic. Of course you loose it all during 4wd sessions.
Currently I've been researching tires to ensure I get the best combination of tread and low weight.

So again, learn to control your pedal foot and you can bring it back up.