Sounds to me like they through a part at it. If your front wheel bearing was bad you would hear it in any t-case mode excluding 4LO. I do think 4HI might amplify it a little and definitely will feel a little different than auto since it is fully locked in. It might even give you the hum you indicated but should not vibrate through the wheel. How many miles?
2005 SE Off Road: JBA LT Silver ceramic coated headers, Cajun B-Pipes with Heavy Metal Cats, Borla Dual SS Cat Back, Uprev Tuned, K&N Air Charger, CSF 3328 Aluminum Radiator, Grille Craft Upper and Lower Grill, Burtman Rear Air Reflector, Manik SS Nerf Bars, Nissan Tow Mirrors, Bilstein HD's, Moog 81085 Rear Coils, AirLift 1000 Rear Springs.