Bumpsteer is where the vehicle steers when the suspension moves, sounds like you're experiencing it with your truck.
If you've lowered it a lot and it still has the standard steering setup, you'll need to either shorten the Pitman arm or make a bracket to move the steering box up on the chassis so that the drag link is more or less parallel to the ground when your truck is normally loaded.
If the drag link makes a big angle to the ground, every time you go over a bump the truck will steer one way on bump and the other on rebound. IMHO, moving the steering box up is best; if you shorten the Pitman arm, it increases the amount you'll need to turn the steering wheel to get full lock. Also cutting and welding steering arms has led to some tragic accidents in the past.
The other thing that might be causing or making thee steering worse is that the castor angle may have changed through lowering your truck, would be worth checking as it's easy to adjust.
Stuart.
