Where do I jack up and put jack stands to get the rear of my 03 xk8 up in the air safely so I can grease my u joints?

 

There are 4 points at the ends of the rocker panels where you can put jack stands.

The procedure to lift the rear of the car with a floor jack is to lift where the rear cross member comes together in the middle. Looking from the rear of the car, you will see two large bolts.

JTIS says to put a block of wood on the jack and lift there. My car is lowered and my jack is not especially low profile, so I have to forego the wood.

 

Drop a hockey puck into the jack saddle. It'll sit only a half inch higher or so.