i'm in a similar boat with my frontier, except its a bit larger, with a V6 and an auto tranny. My best ever tank average is justttt shy of 20mpg. Aside from what you mentioned changing, just trying to ride in the highest gear you can without lagging the engine is another possibility. I dont think pulse and glide is wise in a truck that old nor in a 4x4 in general. I'm also assuming that your tires are 31x10.5, not your rims
if you baby it while driving already, which is how i get my mileage up, then you might have to start looking at other techniques and for sure some mods to get your mileage up especially into the mid 20's range. theres a thread on here with a 2002 ish toyota tacoma crew cab 4x4 that gets in the high 20s with a bit of modification