The vent for the inlet filler tube to the tank has an extension that sticks down about 3"-4" from the top of the tank. When the fuel hits this extension, the vent is closed off, so the fuel going in the main filler tube has to flow against the air trying to come out of the filler tube. That is why the tank fills so slow for the last 4-5 gallons.
The Harpoon mod (named for "Maroon Harpoon" who first suggested this mod) involves dropping the tank and cutting off the vent pipe extentsion flush with the top of the tank. The vent pipe is just PVC, so you can cut it off clean with a PVC pipe cutter.
The mod is extremely easy and alsmost free. Dropping the tank and putting it back is the hard part.
I just did this last Saturday on my 2000. What year is your truck? The later ones (current body style?) only need to have the pickup tube cut off. It's pretty simple. Older vehicles take a different mod. For mine, I took some tie down straps to hold the tank in place as I unbolted it. Then, I unbolted one end of each strap and lowered the tank down low enough to get the pump/fuel lines off. These are held on by a big "nut" like thing, that looks like a giant platic pipe threaded thingy (technical term ). I used a long screwdriver and hammer to turn that "nut" counterclockwise until I could turn it with my hand to remove it. Next, I removed the two tubes going to the fuel fill door, then lowered the tank on down. My pickup is a crew cab short box, I was able to rotate the front strap out of the way, and just bent the back strap. Once I got the tank on the ground, I reacned in with a ratcheting PVC pipe cutter and cut off the tube sticking down in from the small tube. Putting the tank back in the truck is pretty much the reverse of removal. You may find it easier to use a jack to hold the tank in place while you put in the screws.
It goes without saying, I hope, that you should have the tank pretty close to empty before doing this.
Before the mod, the tank would only take 22 gallons or so and then I had to dribble in the last 4-5 gallons. Now it takes 29 gallons before shutting off. I'm happy with it.