You dont have to drop the tank on the first gen expys.You can pull the enitre fuel hat out with tank in vehicle. I know that. Not sure about your year model, but you could check by Take a look over on Drivers side of vehicle above gas tank all the way at the rear. There should be a plastic cap where the lines go in the tank. You will need to remove the fuel lines, unhook the electrical connection, remove the retaining ring. (there is a special tool for this but if your careful and use a blunt headed object(brass drift etc)you can tap it around with a soft mallet hammer, and then pull the fuel hat up. Be careful not to bend the arm on the float too much. Then you can pull the fuel pump fuse and turn key on. check the fuel guage, turn key off, move float to oppisite end and turn key on again. IF you get the same thing each time you need to then undo the yellow wire going to the sender and turn key off and ground the wire. If you get a full reading then, its the sending unit. If not you need to track wiring down to the guage. Its either wiring between tank and guage or guage itself.
I have never replaced a guage, i have replaced quite a few tank sending units.