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First you will need to make sure the brake lights work. This will determine if the brake pedal switch is operating correctly.
Next you will need to disconnect the shifter linkage at the trany. Verify that you can manually move the lever that sticks out of the trany. Verify that it moves through all the gears.
Next remove the steering wheel covers and disconnect the shifter cable from the shift linkage. Manually move the cable to verify that it travels freely.
If all checks out ok, you will have to replace the shifter lever inside the column.
Please let me know if you have any additional questions.