What vintage? There are 2.5 generations--in 2014 there was a mild sheetmetal update, along with an interior update, but which had the same drivetrain, hence the ".5" generation.
First gen was through 2006, and had the 3.4L V6 or 4.7L V8. These did have frame rot, low mpg and other issues. I don't know much about 1st gen.
Second gen was in 2007. The V6 changed to the 4.0, but really, you don't want the V6 in these models. The 4.7L V8 was always a good mill, some think it underpowered in the full sized, and it was always thirsty. In 2010 the 4.7 was replaced by the 4.6, which is in the same engine family as the 5.7. The 3.4/4.7 was a timing belt, while 4.0/4.6/5.7 are timing chains. For the most part, the engines are relatively trouble-free. Toyota seems to have water pump issues across the board, so the 5.7 can need water pumps. Cam tower leaks can occur, and are not trivial from what I understand. The flex fuel 5.7's (which is most of them after a certain date--the 4.6 were not flex) can get stuck thinking they are on E85 which leads to truly abysmal mpg. [The 4.6 ought to have water pump issues, but I don't think I've read of one. Probably because of relatively low take rate on 4.6?]
In general, the 5.7 is the one to go with. It always had the six speed automatic, and the towing package. The 4.7 never got the six speed, while the 4.6 always had the six but could be had in non-tow configuration.
2nd gen weak spots are:
-water pumps
-cam tower oil leaks
-power steering racks after 2010 or so
-an overly complicated traction/VSC setup (three pushes to disable all. Really? it's stupid)
-it's a part time 4x4, no diff in the center
-some do not like the reach for the HVAC in 2nd gen; 2.5gen I think fixed that
-mpg, with many users hovering around 15mpg real world.
-no limited slip nor locker in the rear
-IMO the 18" tires are odd sized. Certainly not cheapie 16"
-many dis the 2nd gen styling; many dislike the overgrown nature
-a horribly expensive air injection system which is well known for failure
-2nd gen seems to have avoided frame rot--but it's a Toyota, so you know rust will be an issue
-front differential. It wasn't all years but for some of them they preset the bearings wrong and it'll eat the diff.
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Myself, I have a 2010 4.6L with tow. IMO, if it was non-tow I wouldn't like it--sometime I think it could stand a bit deeper gearing in the rear to make up for the low-torque engine. I cheaped out with the 4.6, although it does just fine for my pedestrian usage. Right now it's getting about 16-17mpg in cold weather, snow tire usage and me driving about 70mph everyplace. I have eeked out 21+ in summer by very very careful driving.
When I bought mine I decided I did not want AFM/MDS/unproven transmissions/diesel/direct injection. So it's primitive today. But at the time it also was the way to get a > 6' bed with a proper 4 doors (albeit reduced rear leg room, not a problem with kids).
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Would I buy mine again? I dunno. At times the automatic transmission drives me nuts--but any auto is going to do that, as I much prefer stick. But the torque convertor lockup programming is... moronic. The part time transfer case bugs me, as my driveway requires lock to lock turning. It's horrible in snow (snow tires went a very long ways--without them IMO it's best to park this thing). I dislike the throttle tip-in. I never planned to daily drive it, but right now I am, and I find the lumbar support a bit lacking. The dashboard lights dim when I turn on the headlights--which means I have to readjust if I turn them on during the day. Disabling traction is annoying--it's overly complicated.