This sounds strange
I would have thought that if you had replaced your rear diff with a lower ratio, your centre diff would have gone bang by now.
Would be worth checking though, Jack one wheel up on the rear, place a chalk mark on the wheel you have jacked up and one on the rear propshaft, turn the wheel round ten times and count how many revolutions the propshaft turns.
Repeat this for the front and make sure it matches.
I have heard of this happening on series vehicles but never defenders.
Odd
Jim