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.
1992 XJ -- stock (RIP)
2000 XJ -- lots of basic mods. Sold to fellow forum member. I miss it.
2014 JK -- the project is just starting!