1)well for one the 20 inch rims are hurting you, they are more than likely heavier than the stock rims and that will hurt your mpgs your best bet is to stick with the factory rims if you have them.
2)the cold air intake will most likely hurt your mpgs i you have a heavy right foot, and if you put an off the shelf one on it will make a different, cooler sound that will make your foot even heavier, got for a k&n drop in which flows better and call it a day.
3) throttle body spacers make VERY GOOD PAPERWEIGHTS, they dont do much else, if you want to do something along those lines get in touch with dan the fastman http://www.thefastman.com/index.asp
and get a bored out tb
4)if you went up on the exhuast size you lost some backpressure and most likely moved the hp/torque curve higher up in the rpms, not good for efficiency
no onto the other suggestions
1)drive like a granny. in theorey yes, it will work, but IMO if you just drive the speed limit and dont drive like a jackass it will help out enough that its not an issue and you dont have to slow to 55 bc lets face it at 55 on an interstate highway where the limit is 65 you are going to get run off the roae
2)lowering it will decrease drag, but if it is still stock it is going to be expensive, and you will loose some ground clearance which is always good to have if you need to drive in snow (iv driven home pushing snow w/ the bumper of mine a couple times, im NEVER dropping it)
3)yes if you take off the clutch fan and put in an e-fan it will help, however that conversion gets expensive so its up to you what you want to do there are write ups on it in this section so take a look and decide if the expense is worth it for you.
4)hahahaha why would you want to take on a new car payment if this ones gonna dissappear next month? mines paid off, replacing it is NOT an option until it blows up in my face or the repairs cost more than a new vehicle would.(however it is the fastest way to get better gas milage)
5)chances are you have 3.55 gears in the rear, so while the 2.73s would work i wouldnt do it, you will have NO form of acceleration until you get the engine up near the redline, and it WILL hurt your around town gas mileage, it will be a completely different animal than what you have now, just dont do it.(besides i think its like $600 per axle, andin a 4x4 you need to do both)
6)thinner oil theoretically would do it, but i dont trust it to lubricate as thoroughly as what the engine was designed to run, since you are already using fully synthetic i would just keep using the recommended weight and call it a day in that respect.
7)increasing tire pressure will help with mpgs, i run mind around 37 all year round, i keep a tire gauge in there so that when it snows i can let some out for better traction, has always worked for me and saves me a little on gas
for the extra rim, i would go to a junk yard and see what they have bc shipping from anyone on here is going to be ALOT.
and just for the record (it sounds like you have done the tune-up recently) i run a 2002 w/ 4.7 and 4x4 and i was getting 18 back and forth to work this summer about 20 miles each way mostly highway