A wheel alignment is far from one single, simple kind of job, and can indeed be done on all four wheels at once or on just the front wheels. A rear wheel alignment is much more difficult, and even if the mechanic finds that the wheels are out of line with the front, there usually isn’t much he or she can do if the car is a front-wheel drive.
Prior to front-wheel drive’s popularity, starting in the later 80s and taking off from there, only the front-wheels on a vehicle were given alignment. On rear-wheel drives, there is usually no adjustment able to be done. The use of front-wheel drives freed up the rear axle for adjustments to be made.
A four-wheel alignment is beneficial to every car and is always suggested when possible. If the wheels come out of alignment even a fraction of an inch, the difference can cause excessive wear on the tires and greatly reduce handling or cause pulling to one side or the other.
Special alignment jobs are also an option in some cases. For example, the rear axle may be uneven, crossing at an angle making one wheel slightly further ahead of the other. This is called a thrust angle, and may cause the car to drift to one side. A thrust angle is remedied by a thrust angle alignment, in which, instead of straightening the rear axle (a very difficult and expensive job), the front axle is repositioned to match the angle of the rear.