What are the potential damages of running a tire with mis-matched tread life? As far as my little mechanical understanding goes, only when driving in a perfectly straight line, will there be a constant difference in wheel speed which the front differential will counteract for. Unless driving on a straight highway (which there are very few of in the PNW), the diff will have to work anyways, as we rarely ever sit in a perfectly straight line for hours like someone in Texas might. I guess when turning one direction, the diff will also have to counter act for a bigger difference in speed due to mismatched tire diameter, but then turning the other direction, the diff will counteract less speed differential, so they should equal each other out? Feel free to correct my limited knowleadge, especially if there is more to it than just the diffs. I would really appreciate to learn more. Thanks!
You aren't wrong, but it is a bit more complicated than that. On an all wheel drive system we need to consider not just the differential in the axle with the mismatched tire, but also the center diff.
For the tire diameter differences we are discussing here, it is basically a non-issue. But imagine we had a tire/wheel with, just for the sake of example, a 30% smaller circumference. When driving in a straight line the that wheel will be turning 30% more times per mile than the normal sized one.. now technically an open differential is literally designed to allow differences in rotating RPM for the sides of the vehicle, but as those rotational differences get too large we start asking too much of that differential. If one tire was stationary and the other spinning at 50mph.. you can imagine the little spider gears and thrust bushings saying "NOPE" before too long.
Now even without such an extreme RPM delta, over a long enough time, a smaller delta can also be a problem. Where exactly that is depends on the individual parts, fluid condition, all sorts of things. That is one reason toyota just left it open saying "remarkably different" treadwear.
With our center differential, if you have smaller tires on the front than the back, it will be asked to do the same job as an axle differential after the torque makes it out of the transmission. The nice thing here is only one tire on an axle being undersized means any RPM delta between front and rear (aka what the center diff needs to address) is half of that tire's relative undersize... because you still have one full-size tire on that axle still, and that axle diff averages those two.
All of that is in the context of an open differential, meaning no limited-slip functions. Any typical friction-based limited slip system would also be working to fight against a rpm delta from one side to the other.. but our axle diffs are open.. so that's not a concern.
But, our center diff is a Torsen design.. which means our hypothetical 30% difference at the front axle becomes a 15% difference between front and rear at the limited slip center diff... for potentially hours on end?
Fortunately toyota is well-known to over engineer their drivetrains, and OP's relative tire diameter difference is so small, that (IMO, as a judgement call) they aren't to the point of damaging anything, even over the long term. But I guess it comes down to what toyota meant by "remarkably different" treadwear.
An owner that upgrades their primary tires 2-3", but not the spare, then has to install that spare to get home on a lonely stretch of road in Utah? Whole different ball of wax.
OP is in the unfortunate position that avoiding all risk means buying four (if the spare isn't dated out) or five (if it is) new tires, because of one sidewall issue on 30% worn tires. I just can't see toyota having designed these amazing vehicles to be so fragile, personally.
I know I am asking on Ih8mud, but other than "throwing these stoopid 21' set away and putting on some proper 35 in. M/Ts with a 10 foot lift", can I throw in the spare tire which looks to be brand new and never rotated before by the previous owner 
, or will this destroy the full-time 4WD system? Or, should I put this defective tire on the rear axle, where a blow-out would be presumably less catastrophic (btw, I have driven on these tires HARD for about 3k kilometres before noticing this today). I would say the other tires have about 70% of tread life remaining.
