1999 Chevy Blazer fuel guage

At full tank, the fuel guage, both on dash and overhead readout, register properl. At approx 1/2 tank, both readout and guage seem to stop registering correctly. The overhead seems to be dependant on the guage reading. If car stops while engine running, both guages will either read enpty or full and take several minutes of driving before returning to aboput 1/2 tank.
Dealership wants to charge me $50 to diagnose and state that it will either be the instrument panel (yikes) or fuel guage sending unit.
Any suggestions?