What is it?

Mystery hitch mounted device on the back of a pickup truck on Hilton Head Island. Any guesses?

Those fittings are very similar to what we used on trash pumps, so I'm guessing some sort of pump.

Looks like an inline filter or something. 

I see our city's Public Works using these when they are flushing hydrants...but I don't know what it's for?

Protects the truck from speeding & toll cameras while confusing people so that they don't notice the device's true purpose

Our town uses something like that when flushing fire hydrants.  Hose from hydrant to the left side, a 45 or 90* clamps on the other side to divert the water straight down. 

You don't hose down passing traffic.  We need a Woody.

Yes, those are female cam locks - usually all kinds of hoses usually less than 150psi so nothing high pressure. Also aluminum so nothing with chemicals or food, usually water or oil.  

I can do this all day and usually I do.

Squirrel ejector?

Looks like a great way to obscure the license plate, so not legal?

(might be a city vehicle of course, they I suspect at least consider themselves above the law)

In reply to Appleseed :

If you didn't do that I was going to.

I would say valve box or flow meter but the 2 female camlocs are throwing me. What about a foam machine discharge?

The top appears to be a guillotine. I'm also wondering if the face is actually a window. An open grate would just piss all over the road... hopefully somebody will recognize it and make sense of it. 

Looks like an obstructed plate ticket.

Rust on the tailgate indicates it's been spraying out the back with the tailgate down.

It's what was mentioned above, a diffuser when flushing fire hydrants. 
 

Buy it here.

In reply to Steve_Jones :

Bingo. But now I want to understand what is the "diffusing" that is taking place.

edit: apparently it just means making the hose stream less of a hose stream

In reply to OHSCrifle :

Correct, instead of the hose flopping around under pressure, and a stream, it hits the side of the box and tumbles out in a wave instead. I was a sprinkler fitter for a few years, many, many moons ago. 

I wish my local water departments had these. They just blast everyone.