Hi danny, I'm quite familiar with these so may be able to help.

Had a tlt 4x4 do this to me about 6 years ago with a n/s rear wheel. First thing to do is check the wiring loom to the wheel speed sensor, the loom is protected by flexible convaluted conduit that can appear undamaged but if it's had a whack can crush the wires - this I have found has been the cause of 80% of the faults on the 4x4 system. Speed sensor is next possibility.

Have you checked both susmic controllers for fault codes? The red status light flashes morse code if an error is detected. One is behind footwell panel in the cab - this is the main controller, the other is under the cab in the engine bay where the seats bolted - this is the 4x4 controller