Fuel gage/sending unit

[AlSmith]

The fuel guage or sending unit on my 96 Dsl Ram has gone haywire. When I fill up it registers full but shows empty in less than 100 miles. The sending unit, pump and float are one modular unit located in the top of the fuel tank. The entire unit is quite expensive. How do I determine what is broke and how difficult is it to replace this or part of the modular unit? Any help would be appreciated!