I haven't heard of this problem before but I think you might be on the right track thinking it's a sensor.
I would assume the shift motor would have some sort of triggering device to let the ECU know which position it's at & maybe this is working out of position when under load.
From the manual:
Interface to transfer motor inside Transfer Box
(X313)
The Transmission Gearbox Control Unit (Z256) has
a seven wire interface to the Transfer Motor inside
the Transfer Box (X313), consisting of the four motor
position switches, the ground for the switches and
the drives to move the motor one way or the other.
Motor Position Switches–pins 17, 32, 33, 7
The Transmission Gearbox Control Unit (Z256)
reads the position of the motor in the form of a
binary Gray Code.
So the Transmission Gearbox Control Unit (Z256)
can find out in which gear the Transfer Box (X313) is
in and where the motor should move to, to engage
the desired range. If the Transfer Box (X313) moves
outside the normal modes (e.g. Left of High) then the
Transmission Gearbox Control Unit (Z256) can move
the motor back to the correct position.
Motor Position Switch ground–pin 19
The switches on the transfer box motor are
referenced to pin 19, i.e. if there is a problem with
the harness to pin 19 then the Transmission
Gearbox Control Unit (Z256) will not be able to
assess where the transfer motor is and so will not
move the motor.
Motor drive Clockwise (pins 25, 26) and Counter
Clockwise (pins 1, 2)
The Transmission Gearbox Control Unit (Z256)
receives signals from the motor position switches,
the drivers request, the vehicle speed and the
transmission neutral signal and, when the
parameters are correct, powers the motor from one
mode to another via the motor drives. This is actually
a single wire interface for each direction which is
split down to two wires at the Transmission Gearbox
Control Unit (Z256) to help pass the current though
two pins rather than one. If there is a problem with
either the motor drives or the harness, then the
motor will not move and a range change will not be
possible.



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