sounds funny that it stays on FULL....
cuz if the fuel sensor is dislodged then it would read FULL on half of the tank... then Drop suddenly to Empty (not gradule).
all I can say is that Get your self machanic tool then Open up the Fuel tank, Trust me it's not that difficult... exapt doint 1st time would be bit hard...
My guess is the clip that hold the sensor is out of alignments... and if so that's easy to fix if you do it your self....
ps.once you open up the fuel tank you will relise that there are a lots of electrical cables around, on , inside of the tank....
cheers
Joe