The 4.6L 3V engines are very reliable as are the 6R60/6R80 transmissions that come with them. The 2009-2010 V8 models come with a slightly upgraded transmission, the 6R80, that handles more torque than the 6R60 in the 2006-2008 V8 models. I don't think either one has more issues than the other. If a 4th gen V8 model has been well maintained then 200k+ miles from the engine and transmission is fairly common. This drive train is the same one that Ford put in the F150 for several years and in these vehicles getting 200k+ miles from this drive train is very common. I recommend you buy a 2009-2010 V8 model as these have one piece spark plugs that won't break in two when being removed, better insulation on the wiring that won't deteriorate like it can on the 2006-2008 models and the upgraded 6R80 transmission. IMO, if you find a decent condition 4th gen V8 with around 100k miles on it and do a full fluid change (transmission, differentials, transfer case, coolant, brake fluid and power steering). Then change the engine oil every 3k-5k miles, a pan drop and drain for the transmission along with the transfer case drain and refill every 40k miles you will have good odds of seeing 200k+ miles from the drive train without a lot of major issues.
This being said, expect a few wheel bearing changes and other items to deal with like you would on most any other SUV. But the main components of the drive train should be good for quite a while.
