Two-wheel alignment aligns the front wheels, while four-wheel alignment aligns all four wheels. Usually, the latter is not necessary on a big SUV or truck with a solid rear axle, unless it has been in an accident and it might pick up on a bent frame where the rear axle is out of line, causing the vehicle to dog track.
When an auto shop does a two-wheel alignment on a vehicle that has a solid rear axle, the mechanic should also put the alignment heads on the rear wheels because this aligns the front wheels with the rear ones to give the vehicle improved handling.
To do a proper alignment on a vehicle with four-wheel independent suspension (most cars, small SUVs and vans), the mechanic should perform a four-wheel alignment and adjust toe, camber and caster on the front wheels, and both toe and camber on the rear wheels. This allows the vehicle to steer and handle correctly. When the front wheels are straight, the steering wheel should be straight.
If wheel alignment has been performed and your vehicle pulls in one direction or another on a flat road, remember that most roads have a slant or crown on them, so there will be a slight pull. If the steering wheel isn’t straight when the front wheels are or the tires are wearing incorrectly – for example, more worn on one edge – this indicates a problem with the alignment.