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 2" lift -- RIP
2000 XJ 3" lift -- Australian Ford Big Brakes up front, rock rails, ZJ rear discs (with WORKING parking brake), C4x4 rear bumper/tire carrier, Hella H4 headlights, IPF driving lights, OEM skid plates, OME springs/shocks, JKS LCA's, JKS Sway Bar links.