Check to be sure you don't have a dry shaft up front....I would check to make sure my transfer case has not lost any fluid or worn through.remember fellas it's not a case of whether your transfer cases will have pump rub IT IS WHEN!!I don't want to freak you out man,but research ''pump rub'' either here on go on Google typing out ''GM pump rub on Duramax/Allison transfer cases'' and you can read about what causes it quicker than I can type it and probably explain it better....but make no mistake when it goes your truck will NOT BE ABLE to even be operated in 2wd,due to the design of the T case