How to replace a jeep tj fuel tank?

I would be really careful with doing this repair as this is very dangerous. You need to relieve the fuel pressure by pulling the fuel pump fuse and starting the truck. Drain the tank with a siphon pump. Remove any exhaust or other parts that are in the way, loosen the straps and support the tank with a floor jack and a block of wood. There are several hoses, and a few wire connectors to disconnect and you may want to replace those hoses while you're down there. Once you put the new tank in, reconnect the electrical and hoses, raise the tank and strap it back up. Put anything else back together you took off the truck.
Personally, I would take this in to get fixed as this is dangerous work (I did mention that). I you do this anyway, use your head and buy a proper container to hold the gas in , as well as jackstands and a class "B" fire extinguisher. Disconnect the battery and don't smoke or use a drop light under the car, and I will not accept responsibility for this.