Simpleman: Something else you might consider... I have a similar setup to you ('99 Taco pullin' a wide flatbed), and even with plenty of tongue weight, I still get a little bit of sway when things get bumpy at freeway speeds. I'm 90% sure it has to do with poor/old suspension on the front of my truck, along with bad bushings on my sway bar. To get a better idea of the physics of it, think of a dog wagging its tail and the shoulders/head swaying back and forth with it.
I'm upgrading/replacing all of that within a couple weeks, if you want I can let you know how it's holding up.
4 out of 3 people have trouble with fractions.