GM-GMC: 2002 gmc van, gmc van, fuel vapour

Question
when I fill the gas tank the engine bogs when the throttle is pushed about half way to the floor. when the level of the gas tank reaches about 5/8 full the engine bog goes away and full engine power returns.

Answer
I am not sure I have ever heard that before.  There are a couple of possibilities.  It is possible that when the tank is full you are getting liquid fuel drawn into the intake through the EVAP system.  Not likely but it is a possibility.  Try disconnecting the small EVAP line that goes to a large canister located on the frame rail.  There should not be any liquid fuel in that system at all, it is designed to collect the fuel vapour.  I would be inclinded to lower the fuel tank and inspect the fuel pump and sending unit assembly.  You can look for kinked hoses or something along those lines.  I will search some bulletins and repost any updates if I find anything that relates to your concern.
Hope this helps.