2004 Chevy Truck 4WD service

This can be bad because that probably means that your 4wd is trying to engage. I'm not sure if you mean it's doing it on its own or if you're trying to put it in 4wd and it blinks on and off. If it's doing it on its own then you could have a problem with your transfer case. If you're trying to put it in 4wd and it's blinking on and off then try going in reverse and then forward to help it engage.