It is a bit difficult to judge what your noise is without hearing it. noise in just high range points to gearbox rather than rear axle. They are a pretty "clattery" gearbox and final drive anyway but they are very hard to break. I have seen 1400hp put through that back end without giving problems.
You could first try removing the gear selector plate just in front of the clutch pedal and visually checking the gearbox bearings, you could also put a bar between the shafts and see if anything moves.
Another thing to do would be to take the lift off and look into the rear axle, the final drives and dif can be seen. You are going to have to take most of this apart if you find a problem it is not just a simple splitting job.