It is Iron Chlorosis.

For one thing it needs fertilizer. When did you fertilize last? What did you use? And please don't run off willy-nilly before answering this question. By using weed-n-feed you are not making good choices with your fertilizer.

Secondly, in your second picture, there is brown grass. Are you not worried about that, too? Can you take a close-up picture of the brown areas while under cloud cover or other shade? Great pix, by the way.

How much rain have you gotten recently? Did you get a lot of rain right before it turned yellow? Do you know what your soil pH is? Is your soil primarily limestone?

i havent done anything to the grass since posting.

i used Sta-Green weed and feed (Lowes) about 2 months ago.

i have some Vigoro 29-0-4 in the garage, and also bought a 50lb bag of soy meal recently.

the brown area were mostly from last summer's drought.

we got rain atleast once per week during Feb and the 1st half of March. April was fairly dry. i had watered about 3 days before the yellowing started.

i dont know the ph. the top 6-8 inches or so is a dark soil, and underneath is a beige/grey clay.

A close up of the brown grass would rule out a fungal process going on there. It should have filled in already.

Your frequent rain in March and April is the reason for the yellowing. The only thing I've heard of (and used successfully) is a product called greensand. It is a mineral called glauconite (GLAW con ite). It is a little expensive but it is the only thing that works in Texas soils. It comes in 40 pound bags. A starting rate is 20 pounds per 1,000 square feet but I've been a lot happier with 40 pounds per 1,000. I'm not sure how it works but it does. Ironite definitely does not work. If you use greensand now your lawn will likely return to a deep green for the rest of the season. If you do not, the yellow will persist until next spring when all returns to normal.

If you have not used soybean meal before, your first app should be no more than 20 pounds per 1,000 square feet. SBM has a tendency to decompose with an aroma you might not appreciate. It helps to build up the microbial population in the soil that is meant to handle that. That's why you should use small amounts at first. You can work your way up to higher amounts later on. You might even start at 10 pounds.

we've gotten rain the past 3 days. 2 days ago i pruned off a few lower branches off the live oak, thats why there are a lot of green oak leaves on the ground. and i spread out 50lb of soybean meal. we have a corner house, im estimating we have about 1600 sq ft front/side yard.

here's some close up pics taken about 30mins ago.

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Great pictures.

Here's what I would do:
1. Rake up the fallen leaves. I'm talking about all the brown leaves as well as the green ones. Those are holding back the spread of your St Aug and may be contributing to the brown grass there. St Aug seems to be sensitive to airflow and the leaves block it off. You'll get bushels of leaves out of there. If after 3 weeks you see the brown areas spreading, get some ordinary corn meal or cracked corn and scatter that at 20 pounds per 1,000 square feet. The brown area spreading would confirm that it is a fungus and needs to be stopped. Corn meal worked for me every time I tried it. If you have already used a chemical fungicide, then the corn meal won't work, but if you have not, then try it.

2. If you are not picky about spraying a herbicide, get some of this and spot spray the areas that have dichondra (die KON druh) and oxalis (see your 3rd pic for dichondra and 5th for oxalis). Remember it is the black bottle of Spectracide with the purple label. I can find it at Wal-Mart and the box stores. Both of those plants, puny as they are, will knock out St Aug. You don't need to saturate the soil. Just mist the leaves of the weeds. You might need to repeat but give it 2 weeks before you make that decision.

Your soybean meal will help. I'm a little afraid of your app rate. It's going to start to stink on you soon enough. It'll last 2-3 weeks like that.