in the "4x4 service" all they do is drain the front and rear differential fluids, and usually inspect the gears being you have to pull the rear cover of the rear diff to drain. with that they will also replace the gasket or rtv on the rea diff also. i don't remember if they change transfer case fluid or not, i want to say they do, it's been a while since i've done one. depending on when it was last done, i would get a transmission flush, inspect the fluid, if it's kinda brownish in color or smells kinda burnt then get it flushed. Unless you have tranny issues there is no reason why you should have to change your tranny and diff fluids once to twice a year... at the chevy dealership i work for we do 4x4 and tranny flush service at 60,000 mile intervals but it depends on if your hard on it or not, towing alot or not. as far as air... 30-35 psi in tires, driver door panel should state the factory reccomendations on a sticker typically. lubricating suspension is important on these vehicles. those are the major things, and of course tire rotation every 6,000 about and oil change every 3k. if you take it to a dealership, they can do all these services in one package typically and will also include air induction cleaning and direct fuel injection cleaning. (I V style kinda) as far as the air filter, 6k is kinda excessive, i'd go 15k to change it or clean it (if you have an aftermarket)usually unless of course you live in the flippin dust bowl or something ha ha. hope this helps