The spring is part of the compensation valve system for your rear brakes, basically the rear of your Jimny is light so only a small amount of pressure is needed to apply the rear brake before they lock up. With the boot loaded the vavle transfers more braking force to the rear brakes to aid stopping. With the Car boot enmpty there should only just be tension on the spring, ie the slack 'just' taken up. If you have lifted the Jimny this should be adjusted.