My '03 gauge shows full all the time. I immediately thought the sender too. I checked the resistance across the sender terminals and it showed 88 ohms at 2/3 full. I think the sender has a range of 230-30 ohms or close. The trouble shooting chart on Rochester's site says if the resistance is in the proper range, the wiring is grounded between the sender and gauge and the sender should be fine. I haven't traced the wires because they go into the engine compartment and are mixed with a bunch of other wires in a loom. Frustrating to say the least.
I'm sure none of this helps but I had to tell someone.