Depending on the current lift and modifications to the Jeep in subject, one could always do what I did, prior to my SYE upgrade.
This is still not a zero modification solution and some clearance is sacrificed but your net clearance is still higher compared to stock height.
Within the image above, I have a UCF "no bodylift" transfer case skid AND
a 1" transfer case spacer. I was successfully running this combo with the OEM rear driveshaft and the OEM fixed upper and lower control arms and I did not have any vibes.
If your buddy, is looking at replacing his OEM transfer case skid and the Jeep is not lifted, the above may be a viable solution.
Note, with the above combo, I was running a 2" suspension lift and 1.25" bodylift with a 1" motor mount lift (MML). Because of the suspension lift, if I didn't have the MML, I would probably have vibes despite the 1" transfer case drop.
With the installation my SYE, I have since removed the spacer and my skid is up where it should be