Being as the noise disappears on moving the vehicle I'd being looking at things like external fittings.
By pushing on the transfer case you may take the pressure slightly off a tight item or even adding that bit of extra vibration dampening that stops the noise, have you looked at the gear selector lever + the Hi/Lo shift under the centre console, is the cable rubbing the body. Is the engine running at the correct idle speed.
My diesel has a habit of at times idling at 800 rpm a quick tap on the throttle the idle drops to just under 750 rpm, I presume its the throttle pot or the throttle control for the engine sticking, also between 60 mph and 68 mph the gear lever rattles a bit, just drive at 70.
As we all know noises like these can be caused by anything, good luck, it is generally something silly and an easy fix but finding the cause, now that something else.
I also don't think its inside the transfer case, I'd say its a cover rattling or something like that or as stated before could even be loose exhaust internals...