This is interesting.
https://www.google.com/maps/place/Denton+County,+TX/@29.8931669,-98.4102907,393m/data=!3m1!1e3!4m5!3m4!1s0x864dd31493ee045d:0x4355cebb4e6cc8d3!8m2!3d33.1418611!4d-97.179026 (https://www.google.com/maps/place/Denton+County,+TX/@29.8931669,-98.4102907,393m/data=!3m1!1e3!4m5!3m4!1s0x864dd31493ee045d:0x4355cebb4e6cc8d3!8m2!3d33.1418611!4d-97.179026)
In case you don't already know the backstory, I found this article:
http://www.expressnews.com/news/local/article/Feds-pick-site-for-Spring-Branch-s-replacement-5978106.php
EDIT: I see the newspaper is hiding the article behind a paywall...try this link instead:
http://webcache.googleusercontent.com/search?q=cache:FtDwYD8IV3kJ:www.expressnews.com/news/local/article/Feds-pick-site-for-Spring-Branch-s-replacement-5978106.php+&cd=2&hl=en&ct=clnk&gl=us&client=safari
This kind of reminds me of the Bugs Bunny cartoon in which he successfully kept a highway department from building a freeway over his hole and had traffic diverted around it. The episode is called "No Parking Hare" from 1954.
I don't know why they couldn't have re-built that post office on a new location outside of the US 281 ROW. Looking at 6/2016 street view imagery the building appears fairly new, which means it probably has no historical significance. And then there's an RV park behind it, which wouldn't really be affected since it's outside of the frontage road path.
In its current form the highway running around this post office situation is a traffic hazard. It will be made even worse if US-281 is upgraded to a freeway on either side of the site, which is a pretty significant possibility in the years ahead. At some point the post office is going to have to get the hell out of the way.