I literally just bought a used 08. from the people i talked to, they said to stay away from the 05s as they were still working out some kinks with the air suspension etc. 06s a little and shouldnt really worry about it with 07s. that being said, anything can break at anytime. so with a used car a lot goes into the history of the car and how it was driven and cared for.
As far as factory warranty this is my understanding, for it to be certified pre owned (have a warranty). The original purchase limit was 50k miles, if it is under that limit LR dealerships can sell it certified preowned (assuming all else passes) and give you coverage up to 65k. I also think all of the car's transaction history must be with a LR dealership. I think thats right, i could be wrong.
As far as my purchase, I basically looked for one that was fully loaded and in nice condition. I ended up sacrificing on mileage, but for my price range and purpose that seemed fine to me. From my research everyone has said the drivetrains have been reliable. Got to drive it around for the first time today (shipped ebay purchase), everything was great. One thing to look for is the Heavy Duty package, that includes the locking rear diff and a full size spare (a good way to tell if the truck has the HD package). I didnt see it any of the ones i was shopping for and mine doesnt have it.