Ask Mr. Smarty Plants

At least you are in the right place to have post oaks, as Milam County (as near as we can tell from those vegetation maps) is in the Post Oak Savannah of Texas. The trees that you have on your property almost surely grew from their acorns on the spot where they now stand. You can seldom buy a post oak at a tree nursery, because they are nearly impossible to transplant. Its roots are extremely sensitive to disturbance, and the tree is susceptible to oak wilt. Slow-growing and long-lived, it is not often used in landscape situations. Although the post oak is resistant to most diseases, it sometimes succumbs to chestnut blight. 

Without actually seeing the tree, and not being plant pathologists, all we can do is give you some clues to do some detective work to try to figure out what in the environment has changed to cause the problem. You have already said that there was construction in the area to which, as noted above, the roots of the post oak are extremely sensitive. Soil compaction caused by construction materials being stored on the ground over the tree roots, cars or machinery being parked there for shade, and possibly digging for utility lines could all be the culprit. 

USDA Forest Service website on Chestnut Blight specifically mentions that the disease attacks post oaks, but refers to it as "non-lethal" to that tree. However, you could examine the trunk for cankers, which could be causing the bark peel. 

Do you have any idea how old your trees are? They are said to be long-lived, and grow to about 50 ft. tall under normal circumstances. There is always the possibility they have simply reached the end of their lifespan. As in almost all cases for the last two years in Central Texas, we tend to blame many plant problems on our extended drought and high temperatures. It has begun to rain again, at least for now, so hopefully your trees will perk up. If they do leaf back out in the Spring, consider watering the roots, which may extend out two to three times the circumference of the top of the tree, with a lawn-type sprinkler that will get moisture evenly over a lot of the root system.

Finally, you do have to consider the possibility of Oak Wilt. For information on symptoms, read this website Oak Wilt in Texas from the Oak Wilt Partnership, in which the Lady Bird Johnson Wildflower Center is a partner. From the same source, see the article How to Identify and Manage Oak Wilt in Texas. 

Again, not being where we can actually see the tree, we would suggest you get some closer help from the AgriLIFE Texas A&M Extension Office for Milam County. And in answer to your question about fertilizer and pesticides-no and no. A native oak should not need fertilizer, because it is capable of drawing the needed nutrients from the ground, plus you never want to fertilize a plant under stress. And when you have no idea what you're treating, spraying a pesticide would be more damaging than for it to be left alone.

From our Native Plant Image Gallery:

Quercus stellata

Quercus stellata