edit - looks like I left this window open too long before replying and it was all said above^. good post.
you'll find that to be very common...do some searching on here and through 'the google' and you'll find a million threads on it. often times, the sending unit either has a poor ground, isn't connected, or simply failed. they were never terribly reliable on the Jeeps, but over time can break down. sometimes the float can pop off, sometimes the gauge cluster can be burnt out, but I'd look at the sending unit and go from there.