I fought this for a while myself and just recently got it resoved. I put a "new" repop sending unit in mine, and found the brass float was split in about 12 places. I ordered a plastic float from summit and all is well.
If the sending unit lead is disconnected from the gauge (or fuse panel), it should read full. When you say you removed the "ground from the sending unit" are you referring to wire that goes from the sending unit to the back of the cab, or the wire that goes from the fuse panel to the sending unit?
This might help
https://www.fillingstation.com/artic...ugetesting.htm