Local Chysler dealer should be able to set you up.....also make sure ground wire is still on old unit..also check voltage at tank...could be broken wire...had lots of wagoneers cherokees and J trucks...never had a bad sending unit...usually bad ground
If you find the sensor is definitely bad and you can't find one let me know. I have a 1981 J10 I just replaced the plastic tank in. The old tank is busted but I think the sending stuff is fine. I would be happy to help out a fellow J-man.