The handling of a truck is controled by a few factors
- Whell base - length and width
- Ride height
- Relative position to center - length, width and height
These things are determined when a truck is made. There is no after-build adjustments to be done. However, knowing this can help in the selection of which truck you use on what tracks at any given time. For example, a shorter wheel base (boogey van) would probably be more suited to shorter tracks and drags, whereas a longer wheel base (overkill) would probably be better suited to rallies with few tight corners. The trick would be to find the perfect truck for each racing situation. Most people, tho, usually find their favorite and stick with it for all tracks.
To demonstrate how extreme the set up can affect handling, take a look at what some people call RTS, or real truck suspension, which aims at making the body movement more reponsive to braking, acceleration and corning. Unfortunately, they don't race very well.
http://mtm2.com/~trucks/trucks.cgi?s=rts
Another example is small tires. These trucks have to be set up differently in order to give the appearance of normal size tires.
http://mtm2.com/~trucks/trucks.cgi?sort=type&o=d&y=2003
In all cases, they are still affected by the garage settings in the game.