If you've ever done a coolant flush on any vehicle ever, you can do it on a 5th gen 4runner.
The transmission fluid is a process, but seems pretty do-able as a DIY. I actually have the WS fluid on the way right now to do mine ($120 or so in fluid to do a full flush), haven't done it on an auto trans in my life. Done tons of manual trans, but that's like doing a simple oil change.
You can do a simple drain and fill, write up here:
How to Drain/Refill Automatic Transmission | Tacoma World
Or a full flush, write up here:
DIY: Full Flush for Automatic Transmission | Tacoma World
Front diff, transfer case, and rear diff are super simple. Literally pull the fill plug (to make sure you can get it off) pull the drain plug and drain into pan, put drain plug back in with new gasket, pump new fluid until it starts coming out the fill plug, then Plug the fill plug. I used Mobil 1 75w90 in all 3. Some say the transfer case should take the Genuine Toyota 75w fluid. $9 gets you every gasket you need to do.
GENUINE OEM TOYOTA 4RUNNER TACOMA TUNDRA TRANSFER AND DIFFERENTIAL GASKETS SET | eBay
Here's a writeup on the diff fluids:
Front and Rear Diff fluid change
Engine oil is straight forward as well. Hardest part is the oil filter under the skid plate. There's a few good youtube videos, here's one:
YouTube
What's the reasoning for needing a steering rack replacement?