I wouldn't fret too much about the axle shaft. It's really unlikely that you'll break one without re-geared axles and a bunch of weight on taller tires. That's usually the formula. Also - be smart behind the wheel. When you're stuck and it starts to hop or bounce - stop. It's rare that you'll ever bounce your way out of where you're at. And my general experience is that about the 4th bounce is when something breaks. It's the shock load on the axle that causes the failure most times for me.
The front CV axles are pretty easy to swap on the trail. You won't want to drive far, but you could limp a few miles if you really had to. Go very slow. The floppy end of the axle will be bouncing around uncontrolled and could do a lot of damage if you go fast. If you had to run without it - you need to cut the boots off on both ends and remove the axle shaft from both joints. Then you'd also want to leave the broken end of the outer CV still in the hub to hold the wheel bearing together. And you'll need to leave the inner CV in the differential to keep the fluid inside. Then you could drive in 2wd. This isn't ideal - but it would work in a pinch. - also as a side note - tip the vehicle to the opposite side so the gear lube doesn't pour out of the open axle shaft hole when you're swapping them and you can skip draining and re-filling the diff in the dirt. That's not very fun.
Toyota did a fantastic job of changing around the front suspension design in a way that makes it really easy to do. But you'll need to have all the tools to do it. The biggest missing tool from most standard kits would be a socket large enough for the outer nut on that holds the outer shaft in the hub. IIRC it's a 35mm, but a 36mm is more common and works just fine too. You should be able to buy one for under $10.
Unless you're going rock crawling I would probably not be too worried about it. I don't carry a spare axle unless I'm going somewhere that I am going specifically to rock crawl. I've never broken one in this 4runner either. They're way stronger than the old ifs axles. But I've broken enough axles on other vehicles that it's just a nice piece of mind to have. IMO - just buy a cheap replacement axle on Amazon/Ebay for $50 and it'll work to get you home. It's not as strong as the OEM toyota ones - but it's a spare. You could probably watch on youtube how to swap one out. Having the right pry bar to pry the inner side out of the differential is always a PITA. It takes more force than you think.
The same shaft assembly works on both sides. So you only need one.
Also - there's really not very many truly remote areas in north america. I hate giving this advice - but the reality is that if you have all the tools, someone will come along and be able to help you get it swapped out. I've helped a ton of people do trail repairs over the years and had help myself too. It wasn't true 20 years ago, but in today's world there's so many more vehicles out everywhere that having the parts and specialty tools is probably more important than knowing how to do it.
