No one that I know of makes a throttle body spacer for the 3.0. Downey Off-road makes a larger throttle body(bores out your oem one), but it isn't CARB legal here in CA.

When was the last time the spark plugs, cap and rotor, spark plug wires, and air filter changed? When these parts start to get worn, they can cause poor gas mileage, and are the cheapest place to start.

A bad/old O2(oxygen) sensor will also cause poor gas mileage.

What does teh exhaust look like?, did the previous owner put a large hi-flow system on it? An exhaust that is to open will cost these engines their low end torque, causing you to be deeper into the throttle than you should be, reducing your mpg's.

Low tire pressure will also hurt mpg's, as well as carrying extra weight?

A clogged EGR valve will also reduce your gas mileage.

We can help you narrow it down by getting your rigs info, i.e., tranny type, gear size, and tire size. The gear info will be on the door jam plate, in the lower left hand corner by the oem tire size, it should be either G284, G294,G254, or G144.