I have a 96 with the inline 6. Has anyone had an issue with the excess fuel returning to the wrong tank?

 

You have a bad pump in one or the other tank

 

The tank that has the bad pump will be the tank that is filling up with fuel - check valve in that pump is shot. It is also most likely bleeding off fuel pressure which will eventually result in a non-running truck, so better to replace it sooner than later.