3000 lbs?

That's a lotta' Yugo.

That allows weight for me and three fat chicks to take with me boating.

I had a problem last weekend towing the race boat to long island. First time i towed with the truck. The boat kept swayying going down the road. Boat was full of fuel. And have towed the boat behind a bunch of different ford trucks. And never this problem. Now coming home the boat towed good with an empty tank. What could it be?

Worn steering components,
too little toe-in on front end,
Too little air in tires (or maybe too much)
Wrong tires (are they stock?)
Lifted truck?

You clearly have a weight distribution problem. Likely it's too little tongue weight. That's aggreavating whatever is wrong with the truck. Sometimes you'll get sway with too much tongue weight but it's got to be extreme. Some trucks are more sensitive to rear squat than others (squat causes changes to your front end's steering geometry, moreso on IFS trucks)

Yugo's about 1,700. You'd have to take all four.

Is the 4th riding in the middle of the back seat or in the hatch?

Its a brand new 2008 F450. Had 750miles when i left the house.

the little one on the right could ride in my lap!

How much does that race boat weigh?

If you've towed it with similar vehicles in the past and not had any problems, you wouldn't think a truck as robust as the 450 would have much sensitivity to heavy tongue weight. Still, that's pretty much going to have to be the root of the problem if the issue goes away with light tanks- unless you're pulling a raceboat with saddles in the back.

is it squatting much when you drop the tongue onto the hitch ball? 2wd or 4wd?

i like mine