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I wouldn't really shy away from any year, but you should know what you are getting into before you buy used pickup. Injectors were a pretty prevalent problem in the LB7 (01-03.5) Duramax. In my opinion, with a diesel programmer in them, they were able to get the best mileage of all of the Duramax engines so far. You should also make sure that you know the history of the transfer case no matter what year you buy. If you buy a Chevy with close to 100K on it and it has never had any work done to the transfercase, you may want to beware. There is a defect with a retaining clip in the transfercases of all of these 01-06 that are failing left and right and wearing a small hole in the transfercases. The oil will eventually run out and you will need to buy a new TC. I know this because it has happened to my brother and I.
If you need any information, go to dieselplace.com and ask any question you want. There is a wealth of knowledge on that forum, and you will always get good honest advice on any question you may have.
Good luck.