I would triple check the cv axles. I've had them go bad but remain tight. Check the side not making noise as well, most noise in rotating parts is due to or causes vibration which can and will travel into corresponding parts. There are only a select number of possible causes, cv joints in the axleshafts, cv or u_joints in the driveshaft, wheel bearings (sometimes do it in 4×4 long before they do it during normal driving), or differential. Now if your in four wheel drive and doing 40 mph, I'm going to assume you are neither on pavement, gravel, or packed dirt, because just doing that could cause your issue and a few others, among other things running in 4×4 on surfaces with good traction will cause excess wear due to binding when you turn in all of the areas I mentioned plus the transfer case. Full time 4×4 vehicles have a special fluid coupler in the transfer case to differentiate front from back, it slips when you turn because even in a slight turn the front and rear wheels travel different distances. If you are running on packed surfaces at high speeds I would source a junkyard transfer case from a first or second Gen jeep grand Cherokee, it is strong enough to handle a v8, and has a fluid coupler. Just doing that could fix your problem, at least until whatever is making the noise wears more and makes itself obvious. If you want to retain your electronic control for 4x4, I'm sure someone else here can point you to another t-case that may be compatible.
My best guess is cv joints, they take the brunt of everything and are usually the culprit. I would suggest changing the axle you think it is, save the old one for a spare, and see if it quits. They aren't cheep, but if you go off road you will have them go out at some point, it's a given. If the noise quits, save the old one anyway, you may break one and a crappy spare is better than no spare.
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