You need to carefully prise the socket off the ball. the square plastic bit is a dust cover. (socket is plastic and has fingers that can break), might be worth putting it somewhere warm to heat it slightly (airing cupboard, hot water tank)
Mount it is a vice and use 2 levers, carefully.
When you refit the short shift bracket into the rear bracket, make sure it doesn't foul on the frame (mine did on the SR and I had to take it off to file a bit of to provide sufficient clearance)
Regards
Royston