I'm a little surprised to hear people actually opine that you don't need to change fluids on a periodic basis. I'll be the first to tell you that stealerships try to push these things unnecessarily to make profit off unsuspecting customers, so I wouldn't trust anything they say, but it's a good idea to get the facts and science behind your automotive fluids straight. I also think it's incredibly misleading that car manufacturer are starting to label their fluids "lifetime fluids", since no fluids are actually "lifetime." Fluids break down over time and are less effective at lubricating when dirty. Whether it's from dirt and debris that will accumulate (no matter if the system is sealed), or cycles of heat and pressure, your fluids work hard to do their jobs and over time will be less effective at those jobs.
Coolant -- your coolant will not only trap debris, but is also anti-corrosive. I know Toyota says it has "long-life coolant", but it couldn't hurt to do a drain/refill on the coolant, just to get any debris that is in there out, as well as refresh with new chemicals. I think the manufacturer recommendation is something like 100k, but I'd err closer to the side of the traditional 2 yrs/24k mi recommendation.
Transmission -- I drained the tranny fluid this weekend at 67k miles, and it was BLACK. Although it didn't smell burnt yet, I was happy to have it out of there and replaced with new stuff. Transmissions fail when they overheat. When your transmission fluid gets hot, it begins to break down, and you're putting more heat on the transmission, not to mention it begins to be less effective at lubricating the moving parts, plus there could be dirt or metal shavings in there. Good idea to change it, or at least do the old 3-qt drain/fill periodically. As for the comment to avoid transmission flushes, it's a good idea to avoid them if you've gone 200,000 miles and never changed the fluid. That's because all the debris and gunk is caked on to the seals and moving parts and holding them together. If you change it then, you'll "clean" them which can break the debris loose and cause your seals to fail or lodge the debris in the moving parts of your tranny, but really, by that point, it's on its last legs anyway. Best thing is to do the tranny fluid every 50k-60k, depending on driving habits. Note that I've heard Toyota recently changed their tranny recommendations down from 100k to 60k -- if true, what does that tell you about what they're finding with their WS tranny fluid?
PS Fluid -- same idea. You're lubricating your power steering system and all the seals and moving parts that go along with it. Dirty fluid will not lubricate as well. This is an easy procedure to do and does not take a lot of fluid, so good idea to do it. FWIW, I waited 100k miles to do it on my other car and haven't done it on the 4Runner yet, but it'll be in the cards sometime in the 80k mi range. I like the turkey baster method. You don't get all the old fluid out, but it's easy and pretty effective. Plus, I'm always nervous about running ANY fluid pump dry, so I'm not big on the drain method.
Brake fluid -- this one is important. Brake fluid is hygroscopic, which means it absorbs water from the air. This is why you have to throw away a brake fluid container after it's been opened (if you use it within the first month or so after you open the container, it's okay). Even though the brake system is sealed, it's a fact of life that some air (water) will get in there. Water will cause rust and corrosion in your brake system. Notably, brake fluid will absorb water regardless of whether it's used, thus it's important to bleed your brakes on a time schedule rather than a mileage schedule. I've heard 2-3 years is good, you could get 4 years, but I wouldn't go much longer than that. At the very least plan to bleed it every time you do your brakes. Additionally, hot brake fluid can begin to form air bubbles. I know that some people who do auto-cross with their cars will get one day out of brake fluid (or will even have to change their fluid several times during a day of autocross) because they end up boiling the fluid during intensive braking. This creates air in the lines and makes the pedal "squishy" since air compresses. Brake fluid is cheap and bleeding the system is really easy. Spend the 30 minutes every 3-4 years to bleed your brakes. (Also bear in mind that brake fluid will take the paint off your car so be CAREFUL with it.)
Also, I'd recommend changing differential fluid at some point before 100k miles. Re transfer case: I have RWD, so I cannot opine there.
The problem with frequent fluid changes is that you pay for them now but don't get the benefit of them for years. When your car is still going strong on its original transmission and other equipment at 250,000 is where you start to realize that changing things like the tranny, PS, and diff fluid every 50,000 mi was well worth it.
