I'm going with a kind of prairie grass. I yank a handful together with any flower or seed heads it develops and take it to the local county extension agent. They should know right away. And then come back here to tell us what it is.

You are waaaaaay over watering the St Augustine under the tree. And you might be waaaaaay underwatering the St Aug out away from the tree. Oh wait! An hour 3x per week? That's not right either. Watering: Deep and infrequent is the mantra for watering. This is for all turf grass all over the place. Deep means 1 inch all at one time. Put some cat food or tuna cans around the yard, and time how long it takes your sprinkler(s) to fill all the cans. Memorize that time. That will be the time you water from now on. My hose, sprinkler and water pressure takes 8 full hours to fill the cans. Your time will likely be less. I like gentle watering. As for watering frequency, that depends on the temperature. With temps in the 90s, deep water once per week. With temps in the 80s, deep water once every 2 weeks. With temps in the 70s, deep water once every 3 weeks. With temps below 70, deep water once a month. Note that you have to keep up with quickly changing temps in the spring and fall. This deep and infrequent schedule works in Phoenix and in Vermont, so it should work for you. The reason for deep and infrequent is to grow deeper, more drought resistant roots and to allow the soil to dry completely at the surface for several days before watering again. If it rains, reset your calendar to account for the rainfall.

So bringing this one back to life.. With everything waking back up over the past few months, this grass is still around. I've given it a few heavy apps of 2,4d and it seems to stress it but never completely kills it. Do you still think 2,4d is my best bet, besides "roundup"? Should I go heavier with the 2,4d or try something else?

Take a clump to a local landscaper.