There are two units that drop into the tank. One is the sending unit for the gauge, and the other is the fuel pump. Your problem is with the lines, correct? If so, it's the fuel pump unit. Which makes sense too, in that there's really nothing on the sending unit to leak gas. About the only thing that goes on it is the connector that the wire connects to. You get no gauge when that happens.

Can you do without the truck for a few days? Have the B-I-L look at it and see if you can pull it easily. If so, and the cover itself is okay, then replace the lines and save yourself $600! JMO... (From a guy who would be crying at a $600 bill. <img src="/forums/images/graemlins/frown.gif" alt="" />)