Then I'd say the CAD unit, hoses or the t-case switch is the source. You can get one of those posi-lock cable shifters for it. Those are a nice upgrade for a real 4wd truck! Otherwise block the tires, put it in 4wd with the motor running and see if you can hear where it's coming from while laying under it. You might have to be in gear to really hear it so make sure you someone you can trust on the brake pedal. Not someone listed as the beneficiary on your life insurance policy!