If you go with a better flowing cold air intake and exhaust, that could help, but you would need to avoid enjoying the extra hp to see the fuel savings. Other than that, you probably just need to accept that you bought a 4 wheel drive truck with a 5.7L V8 and your mileage is not going to be very good.
The other thread (wherever it is... Half the posts on here are in the wrong place) pretty much says that a throttle body spacer does nothing for our motors because of the specific flow of air pattern. But I agree with @NTX Rebel, I didn't get a 5.7L Hemi 4wd truck for the gas mileage.
Traded a 2014 1500 Express with a 5.7 and 4x4 for the Rebel. I was getting 20+ mpg highway and get 15.7 mpg highway or so wth the Rebel and have just over 2000 miles. The Rebel has bigger tires, more aggressive tires (had Michelin AT2's on the Express) and the Rebel has 3.92 ratio. Have been surprised wth how bad the Rebel's mpgs have been. So now I just change from that screen and try not to think about it.