When l close my driver door and then put the the truck in reverse or drive the ''Driver door open''message appears on the dash.What can l check before l take it to the dealer?

Take it to the dealer, its only a minor problem and should be covered on your warranty.

Just did one 2 weeks ago because when u push the unlock button on the remote the drivers door would not unlock.All wiring checked ok so l replaced the latch assembly.After l changed the latch,the radio would not shut off when u opened the drivers door.Turns out,GM is replacing 2008 door latches with 2009 latches & they r wired different.So l found a bulletin to remedy the wiring problem.GM no longer has switches in the doorway for interior lights,the switch is part of the door latch so ur latch is messed up because its not telling the Body Control Module that u closed the door.
Do not forget to upset the manager at the dealership,because u will probably have to tell him about the wiring bulletin.