Welcome, your statement "drive them into the ground" contradicts "known problems, stuff to look for?", to me stating I drive them into the ground sounds like you are not interested in any maintenance or anything and just want to drive it till it dies. lol
anyway 04 is a good year just do the regular maintenance there really isn't anything to "look out" for except normal stuff that any used car may suffer from general wear & tear. Learning about the particular vehicle and how it functions (or should function)and what features is has or does not have are important so you can determine if it is operating properly or not. Like Mike mentioned above the Denali trim will have air ride you can visually look under the truck and see if the shocks have good working bladders or not, you can squeeze them with your hand to see if they hold air, you can turn the key to run and listen for the air pump to run. you can run the Vin online and see what features it was built with that way you will know what to expect. you can also look over the owners manual online so you can find out how certain things operate and then check them when test driving.