Transfer case and transmission oil levels good? If you have a zerk point on your rear driveshaft, is it greased? If you lift the rear of your jeep off the ground, put blocks in front of the front tires, and let the jeep idle in 1st, does it still make the noise? If yes, you should be able to find it. Make sure you are on flat ground and be very careful. Ebrake off of course.