Yup - Ohm the sender. That will tell you if it's the sender or wiring upstream. I would also try taking it out and doing some left to right and right to left swerves (under control of course) to see if you can jar something back to normal.
I had a similar situation with my 2003 on a couple of occasions within a couple of months. I tapped the bottom of the fuel tank with a rubber mallet under the fuel pickup/sender and after a short while the gauge moved as normal. It has been around a year without the issue reoccurring so I am keeping my fingers crossed.
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