The float could be stuck is one.
FYI, if you do need a new sending unit, the SHOs come from Ford as a sender / pump combo ONLY. It's about $180-$220 IIRC, and the '89's used a different setup than the 90-95. Oh, and the 89 is discontinued, has been for over a year. When I had to replace mine that long ago, there were 7 left at dealerships across the country on the dealer locator service. I ended up having one from Jersey shipped up.
Good luck.