I was driving home today in my 86 jeep cherokee and suddenly the fuel guage went to past full and stayed there.....when I drive it moves from full to empty, back to full, then to half, back to full.....basically it bounces all over the place. When I turn it off, the needle goes past full and stays there. I know I have just under a quarter of a tank, but I hate guessing....any help would be much apppriciated....thanks.

.....I know it's not a third gen, but if it wasn't for this beast, my camaro would be exposed to those harsh canadian winters!

 

Most likely a break in the circuit somewhere between the sending unit (in the tank) and the gauge. Or, like in most old jeeps, the sending unit itself is just about cashed.
There are also many jeep forums to ask or search for answers on this question on which may be of better use to you.