CJ-7 Tim offers good advice. I will also further comment that the tank on a 2000 XJ is made from HDPE (blow molded plastic) and unless it is cracked/leaking, it never gets rusty inside like a steel tank would. Thus, changing the tank is RARELY needed. If the tank actually has contaminants in it (dirt, rocks, etc.), it can be washed out sucessfully and allowed to dry.
WANTED--MJ Comanche to restore. Must be solid, (relatively) rust free chassis. Send me a PM if you have a line on one that is for sale.