Definetly the sending unit. Replaced it in my sons 99 GT. I bought an aftermarket sending unit that didn't fit as well as the original but once installed worked fine.
I would suggest that once you remove the access panel to clean the top of the sender really well to get rid of any dust / dirt that can drop in once its removed. You will need a large pair of snap ring pliers and if you have it the tool for popping off GM fuel lines. I managed with out it.
Do it in a well ventilated area and keep a fire extinguisher handy.
AL
