I've seen caster angle problems plus incorrect toe-in cause this (along with wheel balance, tires not true, tire belt separation, tires worn in a cupped fashion, etc). I can't recall exactly but I think your caster angle ought to be around 2 degrees (negative) and you ought to have about 1/8 inch toe-in or a bit less. Toe-in is not going to cause this by itself but in concert with the wrong caster angle (or a positive caster angle) it can make it worse. If you're curious about your caster angle, here is a way to get an idea of what you have: Drive around in your neighborhood, turning right and left at each intersection. Is it very easy to turn your truck? That could indicate too small of a caster angle. If it is very difficult to turn it, that indicates you have too much caster. Too little caster will be what causes your steering to go all wonky. With too much caster your wheels will be given a lot more downward force on top of that caster angle keeping them going straight and when you turn you will literally be lifting the whole front end up. If you get too much caster and hit a bump on the highway, the steering wheel will usually kick back at you. One more thing about caster: caster must be checked and adjusted with your truck sitting on the ground as it will be driven. Checking it with the front end jacked up is meaningless. If the back end of your truck sits higher than the front, you will lose your caster angle. If the back end of your truck sits lower than the front, you will conversely gain caster angle. I've seen friends of mine create a death machine by putting tall tires/wheels on the back while using small tires/wheels on the front. They think it looks neat but it will make the truck wander around like a drunk billy goat and can induce all manner of steering wobbles and shakes because the weight and angle are not correct and that lets the wheels pretty well go where they will. In Jeeps this problem is very common and is known as the "Jeep Death Wobble."
