fuel gauge moves around and dont read accurate... ideas?

on a 95 peterbilt.. i know my right tank don't work at all because when i flip the switch on the dash to the right tank and the gauge goes all the way down. Now with my left tank, when the truck is stopped the needle seems to stay still (sometimes), but the minute i start driving the needle starts going crazy, it goes down and up to the "fuel level" of the tanks, but it dont read accurate cuz when i fill the tanks it reads a little over half tank even tho im full in both tanks. Any ideas? Sensors? Sending units? where can i start.. Thank you!!

 

also, im gonna do this myself so if you can please help out on how to work on it... thanks!!

 

i mean the needle moves all the way down to empty and goes back to the position where it was, sometimes goes lower, sometimes shakes around...just bouncing all around..

 

how hard is it to remove the sending unit? its not the same as the floater right? i see two things on top of the tank, one is a square metal box with wires and hose coming out of it, the other is a flushed thing with two wires connecting to it, thats the sending unit right? what about the box right on top of it?