woody, what year is your XJ. There are some differences on the pre 91 and the post 91 models. Craig H did a good write up and his home page link has been posted countless times on here, do a search.
He talks to the problems with the ground system and how to improve them. He says all the current goes to the chassis then travels from the firewall to the engine block, then block to Neg of battery. He suggests you add another "bonding" wire from the battery to the chassis. He also talks of cleaning up grounds and applying ox-gard.
I also add that you should attach a bonding wire from the alternator case to the chassis to provide an even better return path when you are working on alternator power. I recommend you try to make this wire a # 4 and less than 24 inches long to get the best benefit.