I've had good luck with Aero tanks in Corona, CA. They have a 26 gallon tank listed for the YJ, but they are able to make anything you want, within reason, if you are willing to pay! I don't have their number, but they advertise widely and you could call 411 in Corona for their number. They built a 33 gallon tank for my Scrambler 2 years ago, put in extra baffling for EFI,and an extra thick bottom plate (for the inevitable taildragging!) for a total of about $450. Hope this helps, good luck.
P.S. If you have the plastic 15 gallon tank, it is easily modified to 20 gallon capacity by removing the extension tube that is in the vent hose from the filler neck. It is easily removed if you can get the smaller hose off from the tank, that comes down from the filler neck, and with a large tap (approx 5/8"-3/4")screwed partially into the tank tube, you can extract the vent hose extension present in the 15 gallon tank. This modification allows the tank to fill to the same level as the plastic 20gallon tank! If you can't get the extension tube out by pulling on the tap, remove the tank,(easy if you are nearly empty) remove the sender unit/pump module and you can reach inside the tank and pull out the extension tube with your hand (wear gloves if your hands are sensitive to gas) It's advisable to have a new gasket handy, as the old one is usually torn or cracked.
