A 2008 with only 40k on it? Wow, nice find. By 2008 there weren't really any "known issues" as far as I can tell. Some of the early V6's had head gasket issues and I've heard some of the 03-04 models had problems with the dashboard cracking as well.
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Biggest thing (especially on a low mileage one like that) is MAKE SURE THE 4WD WORKS! Test EVERY mode. If it's a V6, test it in 2wd (which it should be in now), 4hi unlocked, 4 hi locked, 4 lo unlocked and 4 lo locked. If it's a V8 it will not have a 2wd position but it will have the other 4 modes so make sure you test them all.
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If it fails to go from 2wd to 4wd (the dash lights just keep blinking), DO NOT buy it unless you get a promise in writing to have it fixed. This is a $2500 problem and if you don't get it in WRITING that it is the seller's responsibility to fix, you will end up paying for it.
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From what I've seen on the T4R.org forum, this is most commonly caused by "mall crawlers" that have never been off pavement and have never (or rarely) been put into 4wd. Essentially the electronic 4wd controls are a weak point on these vehicles and it's a "use it or lose it" proposition.
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NOTE: Before someone jumps in with a "I've had my 4runner/Taco for 100000000000000 miles and never had an issue" understand that when I say "weak point" I mean relatively speaking. IOW the electronic 4wd controls on these vehicles are pretty darn reliable overall and there's no reason to expect them not to work. But on some vehicles that have not been "exercised", especially in wet/salty parts of the country, there have been problems with the 4wd failing to engage because the mechanism has gotten corroded and/or has not been used often (or at all.) So make sure you test it when you are test driving.
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Also, just an FYI, the vehicle has to be in NEUTRAL in order to go from 4hi to 4lo. If you have it in any gear other than Neutral, you will hear and angry "beeping" and you will wonder what's wrong. Well, what's wrong is you aren't in neutral. The same goes for when you are going from 4lo to 4hi.