An addendum to the prior posts: If your fuel lines and brake lines are rusty and you replace them, make sure you drop the tank and replace the sending unit as well. The factory fuel lines have a rubber section that is crimped onto an additional length of steel line that connects to the sending unit outlets. The other day after filling the truck up with diesel I saw a puddle under the front of the tank where it was running down from the topside. Looking through the fenderwell with a flashlight we could see that the outlets were corroded so I ordered a new sending unit for $70 from rockauto.com. We dropped the tank and this is what the sending unit looked like. We were also not happy with how the tank looked so I ordered a new one from rockauto which should be here next week. Lots of crud was in the bottom and after checking with a mirror inside we saw corrosion on the inside ceiling of the tank.
Here's the old sending unit:
