And you're sure it's shifted all the way out of 4wheel high Andy? Could it possibly be in 4wd?
Of course, easy to tell if you can just reach under and spin the front driveshaft, it's not in 4wd.
What about any body lift? Could the shifter be touching sheet metal under the boot?
Assuming it's a stock shifter that is? Not a twin-stick or something like that.
Checked that the rear pinion yoke nut isn't coming loose?
And lastly, the driveshaft will sometimes send strange noises up into the rest of the drivetrain and make it sound like one of the bearboxes. Every time I've run across a bad u-joint or yoke, it's been a different sound.
One possible test (though not definitive) would be to remove the rear shaft entirely and drive around in 4-wheel high range to see if the sound goes away or changes.
Good luck. A t-case rebuild is easy and relatively inexpensive as these things go. But it's still a hassle that would be nice to avoid.
Paul