Buy an extra seal. Put the seal in the freezer so it shrinks a little and then pull it out and press the seal right in, don't wait for it to thaw. A lot of us ruin the seal on the first time doing this repair. Since you are doing both sides, 3 seals should be fine as you'll get the hang of it the second time.

Anyone have an explanation for why this is so common? Is this even with the proper tool? I mean I will likely get three regardless but would love to know even more why it happens.

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It's a tight fit so most people don't get it in square and then mess it up trying to get it square in there. Take the seal to lowe's/home depot/etc and find a pvc pipe or fitting that is the correct size to use as an install tool to hammer on.

Along with not having the right sized driver, the seal itself is very thin. The metal ring is very easy to bend so one wrong tap with a hammer and it's deformed and doesn't seal properly. On top of that the rubber ring is also thin and when removing the rear axle if the splines are dragged across the rubber it can rip. A very fragile part considering the amount of force needed to install it.

It's due to operator error when the person is using the correct seal driver tool. Unless you're incredibly lucky, your first hammer strike against your seal driver isn't going to be pushing the seal in perfectly square to the axle housing. When this happens, a common error people make is lifting the seal driver puck away from being perfectly flat against the seal face and striking the seal at an angle to try to straighten the seal driving in the axle housing. When the person does this, they are now applying force to the seal with one edge of the driver puck. This allows the force being applied with the hammer strike to dent the seal and you've officially screwed up.

The key to driving in the seal without damage is not paying too much attention to how squarely the seal is driving in. If you keep the driver puck flat against the seal while delivering hammer strikes, the seal will eventually straighten out and you'll get it fully seated without damaging it.

The reason I know this is I've damaged more than one seal. Every time it happened, I wasn't being conscientious enough to keep the driver puck square to the seal.

EXACTLY what Tim said + use a heavy hammer like a sledge hammer. Once everything is lined up, all you need is just a one good blow to the driver tool from the sledge hammer.

Good to hear it from the mouth of Timmy, thanks for responding bud, your 2 videos on this job are invaluable. Thankfully I'm also doing it with someone who has done this job on a Tundra and a LC 100 series (and has also watched your videos) and he has the proper seal drivers. He's letting me take the driver's seat on this job as much as I feel comfortable and getting a clear reason for why the seal driving gets screwed up makes me much more confident.

He has the Donald tool although not the 4runner bearing driver but we are going to use some schedule 40 steel pipe for that it has an ID of 41mm so it's almost perfect for this job. He also has a lift so this job should be easy mode getting things in and out of the axle housing. Great weather tomorrow too so I'm going to throw on my coveralls (I think that one thing was one of my best investments so far lol) and get er done in the morning.

Also went ahead and did the breather extension mod and found that my old breather was plugged up so I now know with certainty why this happened. Actually found that I could route the hose from the gas cap gasket down that filler pipe cover without having to take it off so that was a bonus not having to pop off those clips that are often not reusable.

Good luck with the job.

Well I finally got both sides done on this. First side was done a month ago. I tried to follow Tim's advice but I screwed up the first seal of course. I was rather confused and the guy who's shop I was using basically said you just have to get a feel for it and actually got the seal in using the exact way you said not to do it Tim. I did the other side today and had the same result. The way I did it was with the driver and a light rubber hammer and very light tapping and actually working my around tapping down the higher points on the seal. As far as I could tell I didn't dent or deform it (I guess time will tell and maybe I did screw it up though). Other than the magic finessing of the seal everything went great though and my seals are now riding on the dead center of my flipped retainers. First side a month ago took me a whopping 8 hours but this side took me only 4.5 and a good chunk of that was spent putting the brakes back together (I hate drum brakes so much ><). Very nice to now have the peace of mind that I shouldn't have any issues with my bearings for quite a while. I'll pull the ABS sensor every once in a while to make sure things are still looking good since I know some do report leaks coming back.

Well, you found a technique that works for you and that's all that counts. In my experience, I ended up damaging the seal when I'd pull the driver away from being square to the seal.

Yeah and just to be clear not knocking on you Tim. The guy who helped me basically expressed that everyone has to kind of learn their own way and that there's no way to learn besides screwing one up.

This is how most mechanics learn and especially how all auto body folks do too.
We learn from our mistakes. Now if a person keeps doing the same thing over and over screwing it up. They are not learning from their mistakes. :-)

There are always several ways to do the same thing. You find a method that works for you. While this method maybe to different from someone else. If the end result is the same that is great.