Welcome JatMat.
I would start with the tire pressure, go to the maximum listed on the tire.
Try coasting down a hill in neutral, without apply the brakes. If is slows or stays steady there could be a brake shoe/pad dragging or a bad wheel bearing.
Is the a check engine light on? Some of the Auto parts places near me offer to check your engine codes for free.
Just a few low cost suggestions, maybe others on Ecomodder will have more.
Here's a link to the EPA site, shows 19 MPG combined city and highway for your truck. You can do better.
Fuel Economy of the 1996 Nissan Truck 2WD