Smearin

I have a 00 trans am ws6.... When I fill up with a full tank of gas, and the car sits... gas starts to drip off the heat shield over the muffler.. I checked the fill tube gasket around the tank and it doesnt seem to be leaking gas..
it only does this when i have a full tank... between 3/4 and full tank.

The only other thing i could think of is the fuel sender unit leaking..

thanks