This article is from the The Monster Truck FAQ, by Jeff Hawkins with numerous contributions by others.
A. No, the rear wheels are not hooked to the steering wheel. On most
Monster Trucks there is a toggle switch mounted where the driver
can operate it with his free hand. The center is off and moving
the toggle in either direction cause the rear wheels to steer more
left or right.
Many of these steering systems also have a second switch to select
a automatic centering capability. When the driver removes pressure
on the steering toggle the wheels will return to a centered
(straight ahead) position.
Finally, when side by side racing many drivers mechanically lock the
rear steering at center position by bolting on a locking bar or
aligning a hole in a locking tab on the steering tie rod with a
corresponding hole on the rear housing.