Use the following procedure to replace the wiper arm on your 2000 GMC Jimmy.
1) Start with the wipers in the park position. Mark the position of the original arm on the windshield with a piece of tape (if you can).
2) remove the cap and nut securing the wiper arm to the wiper transmission.
3) Rock the wiper arm perpendicular to its travel to disengage it from the transmission.
4) Once the arm is off, clean the splines on the wiper transmission with a soft wire brush.
5) Install the new wiper arm, aligning it with the tape mark on the windshield.
6) Secure the arm with the mounting nut and install the cover.