I've been experiencing rapid fuel gage drop with plenty of fuel left in the tank. I just sort of thought it was the main sending unit so I just replaced it. I'm still experiecning issues with the fuel guage. Is there a way to debug the secondary unit without removal? How about other things that could cause this? Also notice that my gauge is not going to the top when filled.

Could be the wiring. Check for corrosion and loose connections back at the tank area. Some folks have posted good results with fuel system cleaner. If you do a search you should find some info.