Just like to say welcome! I love seeing more and more SUV owners joining this group. Looks like there's plenty of room for improvement with the GMC Jimmy. If you do a decent amount of highway aero improvements would be a good place to start otherwise ditch as much weight as possible and work on the aero later.
A few quick/low cost aero mods
Roof racks a great start - Sounds like you took care of that already
Grill block - Partial block is easy/cheap.
Block tow hook holes - Noticed most jimmy's have them.
If you've got mud flaps - Ditch'em
fill in any gaps around lights/Panels - Weather striping I hear works great for this
More advanced/expensive aero mods
Wheel covers
Rear wheel skirts
extend the air dam down
Skid plate - Picked up up about 2mpg highway on mine
cover as much under body as possible - With 4x4's it's hard to do one smooth panel
Weight reduction
Ditch spare tire/Jack - Got AAA?
Any extra tools, oil, etc
Seats - If you're the only one in the car most of the time.
More expensive weight reduction
Lighter rims
Lighter Brake rotors
Lighter Flywheel
If you'd like to see what I've done on my Cherokee so far check out my post
here.
Feel free to add to the list guys.