first thing to do is get that k&n filter off that is actually causing problems.the oil gets into the throttle body and gums up stuff in there and makes its way to the engine along with dirt.That filter will do nothing for that engine as far as gains.

Now on to the fuel milage. You wont get much with a full time 4wd in the first place.you can possibly increase it by checking all your tire pressures and make sure they are at 35 psi.clean your throttle body,i am sure it is gunked up. change all your oils engine, front and rear diff,transfer case,and trans. those things will help but you still may not see very big gains. also check your egr valve hoses, sometimes they get carbon in them and cause problems.make sure your O2 sensor is working properly,they can send false info to the computer giving you poor milage.
as for the springs I don't know, personally i would stick with stuff that belongs.call it paranoia,safety or whatever , i just don't like the idea of putting something designed for one vehicle on another like that.