The problem is that once the lawn was ruined by the drought, it was thin and the crabgrass moved in. There is a cure, though. I suggested this on another forum and one of the guys took it up as a challenge. It worked like magic. Here are the steps.
Step 1. Spray the crabgrass with a mix of water, soap, and molasses (2 teaspoons of baby wash and 1 teaspoon of molasses to a pint of water.) The purpose is to wet the leaves of the crabgrass. Crabgrass leaves are hydrophobic so you need soapy/sugary water to make the next step work.
Step 2. Dust the crabgrass with baking soda. Put some baking soda into a sock and tap the sides of the sock to filter out a very fine dusting of baking soda. Don't try this on a windy day. You might breathe in the baking soda or get it in your eyes (the change in pH of your tears burns your eyes). If you get it in your eyes, just rinse them or apply "tears" type eye drops.
The reason this works is that St Augustine can tolerate a LOT of salt (sodium). Other grasses cannot. The weeds should die completely within 48 hours. Watch the video to see how he did it and what the results were. He had more St Augustine damage than I have seen but I think he over applied the baking soda. Still, the results are pretty remarkable. On the forum where he originally posted this message, he posted a picture 30 days after the test and the St Aug was filling in nicely.
Click here for video link Kill Crabgrass in St Augustine