I have an 04' dakota 4x4 with the 4.7L. I noticed over the past few months that there is a humming/whining sound when driving in 4high. It is loudest at about 35 mph and does not change when I shift to neutral at that speed. It does not seem to be tied to load on the engine or RPM, but rather just speed. It also feels like there's a little extra drag. It stays for several minutes when I switch it back to 2WD. After a while, it will go away and will drive normally in 2WD. About a year ago I replaced the front hubs/bearings. Within the past 2 years I have also done the tie rods and ball joints. The tires are the same as they've been for a long time with even wear. Because of the sound I just replaced the front diff fluid, transfer case fluid and topped off the rear diff fluid (It was just changed 2 years ago). I pulled and tugged on every drive shaft, U joint and CV joint I could find and didn't find anything loose. All the rubber boots are in good shape. My guess is that something in the transfer case is dragging. Any other suggestions before I give up and take it in?