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I found the biggest single aerodynamic improvement was from a grill block (watch that you don't overheat, though - I ended up having to cut a 6" diameter hole in the middle of it). I think that one mod was worth about 1.4 MPG.
Of course, decreasing my speed had an even bigger impact. At 40mph, I get up to 5 MPG more than at 60. And the increase was fairly linear, all the way from 40 MPH to 80 MPH. At 35, it's worse, though. 40 is the best, where I actually get 30 MPG or better!!! No joke!!!
Other improvements: taking the roof rack off was good for just under 1/2 MPG improvement. Folding the mirrors back flush with the sides was only worth 0.1 MPG, and adding cardboard "fender skirts" to the back wheel openings was negligible. - I tested it 3 times, once with a miniscule improvement (less than 0.1 MPG), once with a miniscule DECREASE (still less than 0.1 MPG), and once with virtually no change. Pumping up all the tires to 60 PSI (from 45) was worth about one MPG if I remember right...
But my car is a 2000 Jimmy, which is, I think, a different body style and engine than yours. I had an '85 Jimmy before this one, which was the "square" one. Didn't experiment with aero mods on it, though.
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