In my opinion the more tension on the spring the more braking to the rear . the spring in the photo looks tight . Is the arm coming off the adjustment screw as far as possible from the end re can not move any closer to the spring. if so then the rear brakes have full power which could be causing them to lock . Forget about the spring length and adjust the tension via the elongated hole in bracket connected to the diff try it in the middle of the adjusting pin do not turn pin as this is factory set. Is your vehicle loaded or unloaded if unloaded or light the arm may need to be closer to the end of the adjusting screw. When I did my ute I placed a jack under tow ball and lifted weight off the axle the spring depressed meaning less breaking needed as less weight when lowered the spring tightened meaning more weight which would require more rear braking just a thought .
