Is it a regular cab, or extended?
If you're due for new rubber soon, consider making a size change. Use a
tire calculator like this one to see what works best.
Also, what rear axle ratio do you have? If you're unsure, check the sticker in your glovebox. Somewhere will be a code (mine was in upper right corner) that tells you:
GU4=3.08
GU5=3.23
GU6=3.42
GT4=3.73
GT5=4.10
HC4=4.56
Mine said a GT4, which means I have 3.73s out back.
You are a world ahead of me as far as base vehicle is concerned. I have the 4.3L V6 (vin Z, so it's a TBI), the automatic transmission, and the 3.73s. Plus, I have the horrid OBD1.5, which is more or less useless for instrumentation purposes, but you have the full arsenal that OBD2 provides. If you're committed to getting great MPG on a highway, I have no doubt that you could achieve 35MPG with practice and a few changes.