If you're comfortable replacing brakes on your truck, you can totally do most of these things.
Do have the plugs and wires replaced. There is no manual idle or ignition timing adjustment to be made on your truck - it has an entirely computerized ignition system.
Pop the hood and look at the belt(there's only one belt). Where it goes over the water pump pulley(smooth one toward the center), it will be "inside out", so you can see the ribs and any cracks will be evident. If there are any cracks, or if the belt has never been replaced, have it replaced. This you can do by yourself if you have a 3/8"-drive ratchet with a long handle. Five minutes' work.
Probably do the radiator hoses. This, too, you can do yourself, but it will be messy. Might be worth paying a shop and getting the coolant flushed at the same time, just to avoid having to catch and clean up the coolant.
Don't bother with a power steering flush.
Replace the fuel filter if it hasn't been replaced before. If you don't mind getting under the truck, this is usually very quick and simple(some vehicles require special tools, but I don't believe GMs are among them), and if you do happen to spill, gasoline evaporates :)
Cleaning the air intake is a ten-minute job at most. You can do it yourself: all they would do is remove the intake ducts to expose the throttle body, open it and clean the butterfly valve and throttle bore. Just use a little bit of carburetor cleaner and wipe until it's clean. If you don't want to do it yourself, get it done, but NOT for $57. We used to charge $11 unless the car had multiple throttle bodies or ridiculous ductwork.
I'd say get a transmission fluid exchange(or at least drain & fill), as others have said, only if you have had one done before. If not, let it be.
The differential fluid replacements are expensive unless your truck requires some magical synthetic fluid. We used to do them for $45 an axle for standard gear oil, or as much as $90 for synthetic.
Newer GM transfer cases, as I recall, require an expensive synthetic fluid which is available only from the dealer. You only need a few quarts, though. Shouldn't be expensive.
If you're just wanting basic maintenance done, I would solicit quotes from a couple more shops(avoid chains if possible, you never really know what you're getting into). These guys sound expensive, and the whole idle/timing thing makes me itch - that hasn't been a consideration in probably twenty years.
