I think is sounds like a quite jake brake on a semi truck, mine has it and so do lots of others. It is from lifting is and it is the driveshaft being at to much of an angle that causes it. I bought a set of 3 adjustable rear links to help correct this problem, other have installed them and say the noise mostly goes away but not entirely so I am going to see how much it helps when I install my adjustable links. If it does not correct the problem I may install a double cardan joint to correct the angle of the drive shaft.
The short wheel base Jeeps have this issue too, some are even shorter than the VX so they have issues even lifting a couple of inches http://www.morris4x4center.com/knowl...base-chart.htm
This Jeep forum has a good thread about the different kinds of drive shaft modifications to correct the angle problem. http://www.jeepforum.com/forum/f9/wh...nt-one-854790/