When you touch the power to gnd it's the same as the float going down in the tank. As Tiny said above, default is max resistance and gauge shows full. That's what you see when nothing is attached to anything. You said before that the gauge moved when you moved the sender's arm up and down when you had it out of the tank If it went to full when arm was up and moved to empty with arm going down I don't see there's a problem. Any loose connection during the year you've owned the truck could cause the "Full" condition and you "fixed" it while removing the sender and testing it. I say press on!
