In the last episode of the series, the one where Bobby joins the college meat-inspecting team, it's revealed that Boomhauer is, in fact, a Texas Ranger. At no other point in the series, is any mention made of this. It seems rather odd, as Hank, Dale, and Bill will often talk about their work with each other, and yet no one seems interested in Boomhauer's law-enforcement career, nor, do they ever mention him having a job at all. Until the final reveal, it seemed like Boomhauer was free all hours of the day to chase women and sunbathe. Why would Boomhauer's closest friends not ever question or discuss his job? especially considering the numerous times the characters have broken the law, such as stealing a tank from the Army base? It's simple; they have no idea he's a Texas Ranger. And they have no idea, because he's undercover.
The next natural question s why would he be undercover in sleepy Arlen, TX? Again, there's a simple answer; Dale. Dale makes no secret of his opposition to the government. He's been a member of at least one local militia movement, has trafficked illegal weapons, and seems to have contact with international criminals (as evidenced when he speaks in Russian to the shady guy about buying a Walrus). So, frankly, it would be alarming if Dale wasn't under surveillance.
Also, look at the layout of the neighborhood. It can be kind of hard to nail down exactly how the homes are laid out, but it is clear that Boomhauer's backyard backs to the alley, as does Dale's, presenting a clear method by which Boomhauer can quietly observe Dale's activities.
In flashbacks throughout the series, the four main characters, Dale, Hank, Boomhauer, and Bill, are shown to have grown up together. This may seem to work against my theory, but I actually think it works in favor. Boomhauer was specifically chosen for this assignment because he already had a relationship with Dale, and no one would view him as an 'outsider' within his own hometown. Boomhauer's superiors chose him so they would not have to waste time and resources getting another Ranger close to Dale. It had already been done. So they sent him back to Arlen, likely with a cover story to give to his childhood friends about why he was suddenly back in town.
Boomhauer, however, knows Dale well, and understands that Dale is too much of a buffoon to ever be a real threat to the government. So he accepts the assignment, knowing full-well he'll likely never have to take action against his old friend. Instead, he lives out his bachelor lifestyle, and may in fact even be protecting Dale by continuously sending reports to his boss saying he needs more time to collect evidence, and therefore prevents authorities from ever taking Dale down. And no one on Rainey St. Will ever likely know.