I would change the ATF after 12+ years, even if it only has 40k miles. Might as well upgrade to a Dex-VI fluid while you're at it, as it should improve cold shifting and maybe fuel economy a bit. You will probably get some recommendations from others for an Amsoil full syn ATF, which will probably give you the same benefits.
Diffs should be fine, but it's not overly difficult to drain and fill them, so you might as well. I went with Renewable Lubricants LS Gear Oil (full syn) in both the front and rear diffs in my Burb after a recommendation for it here and comparing it's specs to other 75W-90 gear oils. It is available for a decent price on Amazon. You will also get recommendations for Amsoil (Severe Gear, probably) from others here.
You should also change out the transfer case fluid. I believe it is a 30k or 40k interval. Check to see what make/model of transfer case you have, as some only take the GM Auto-Trak II fluid, which you can get at the dealer or O'Reilly Auto. Again, you will probably get several Amsoil recommendations if it can take something other than Auto-Trak II.