How can l rectify my four wheel gear? It can't change from 4x4 low.

The only way your going to fix this is have someone with a up scale scan tool check an see what DTC's are stored in the ABS an engine modules an the 4x4 system ! The O2 sensor has nothing to do with the 4x4 ! An the ABS has nothing to do with the 4x4 not working either !
Four-Wheel Drive Systems - Electronic Shift Inspection and Verification - Electronic Shift

1. Verify the customer concern.

1. Visually inspect for obvious signs of mechanical or electrical damage.

• Axle shafts and universal joints
• Driveshaft and universal joints
• Fluid leaks
• Matching tire size

• Battery junction box (BJB) fuses:
  • 17 (20A)
  • 18 (20A)
• Central junction box (CJB) fuse 12 (5A).
• 4WD control module
• Wiring harness
• MSS
• Gearmotor encoder assembly
• Connector(s)
• Circuitry

1. If an obvious cause for an observed or reported concern is found, correct the cause (if possible) before proceeding to the next step.

1. If the concern is not visually evident, connect the diagnostic tool to the data link connector and select the vehicle to be tested from the diagnostic tool menu. If the diagnostic tool does not communicate with the vehicle:

• check that the program card is correctly installed.
• check the connections to the vehicle.
• check the ignition switch position.

1. If the diagnostic tool still does not communicate with the vehicle, refer to the diagnostic tool operating manual.

1. Carry out the diagnostic tool data link test. If the diagnostic tool responds with:

• SCP+, SCP-, UBP or ISO circuit fault; all electronic control units no response/not equipped, refer to Section 418-00 .
• No response/not equipped for 4WD control module, Go To Pinpoint Test B .
• System passed, retrieve and record the continuous diagnostic trouble codes (DTCs), erase the continuous DTCs and carry out the 4WD control module self-test.

1. If the DTCs retrieved are related to the concern, go to the 4WD Control Module Diagnostic Trouble Code (DTC) Index - Electronic Shift to continue diagnostics.
2. This is going to be way over your hear , unless you fix these systems on a regular bases ! An not to mention the tools involved , do yourself a favor an take it to a shop that has experience in these matters an save yourself time an money !