Truck Repair: 2000 Chev. Silv. 1500 LS gauge/light problem, chevy silverado, fuel gauge

Question
I have a 2000 Chevy Silverado 1500 4WD LS Short-bed/Single-cab.  My fuel gauge bounces from E to F and everywhere in between whenever it feels like it.  The tank can be full, and the panel will still say Low Fuel and eventually after running a while, the needle slowly makes it's way to full or somewhere in between.  What is the deal?  
Also, another problem is it keeps saying Service 4WD even though I've had it serviced.  I notice that the lights on the Automatic Transfer Case switches don't always come on.  Is it telling me to service it because the button lights are faulty.  The 4WD works fine by the way.  Thank you for your help.
-Tom  

Answer
Hi Tom,
Sounds like a ground problem.
Make sure there is a good ground wire from the engine to the firewall.
There needs to be a good ground from the body to the frame also.
Then up under the dash, above the parking brake pedal probably, find the ground bus connector, and make sure all the ground connections are tight. Especially any ground wires from the instrument cluster.

Van