Tune up is the way to go. But the vacuum gauge idea is also not a bad one to be truthful if you are going for max fuel economy. Also vacuum line leaks kill performance and fuel mileage so its not a bad idea by any means.

The 2.8/3.1 are good motors and can get some decent MPG when taken care of but they are not performance motors and i dont think the factory intake is exactly starving them for air.