The weed and feeds, as well as services like Trugreen, are simply playing the averages with what nutrients are needed. So those products and services may work for the average lawn, but might not work for yours.
A soil test will let you know exactly what your soil/lawn needs. Find your local extension office. They should offer the soil tests for a few dollars ($6 maybe?), and will give you all the directions you need for how to take the sample.
Thank you all for your responses and helpful advice!