On the passenger side of gearbox the low range protection valve can be located, the airline connections from the valve to the range cylinder at the rear of the gearbox must be interchanged. now only the low 4 gears should be available. This is on a ZF 8s151 gearbox range change not a splitter gearbox. Don't know if this will help you but we struggle on cold mornings. Sounds like to me your valve might need replacing.
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