Not sure this will be of any help since I drive an E70 but your explanation sounds remarkably similar to my fault.
Long story short; the ATC500 CONTROL MODULE (it is responsible for controlling the transfer case, among other things) in my vehicle became increasingly unreliable after I discovered it had gotten wet. It ultimately failed completely which resulted in failure of multiple components (EBD, ABS, DSC, 4x4, e-brake, wheel speed sensors... And probably some I can't recall). I was lucky enough to have found a working unit in a wrecked E70 at my local scrap yard.

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