Russ,
Use a large flat screwdriver and a rubber mallet. Place the screwdriver on one of the tabs on the outer ring of the cap on the sending unit. Tap it clockwise (if memory serves me right) it should move slightly, about an inch, and then you should be able to remove the ring and sending unit. Sometimes the unit is stuck to the tank, so you may need to pry it off.
Check the sending units float (for leaks) and filter screen on the pick-up end of the unit.