It is called Super Duper Sumos, and according to Wikipedia it aired on Nickelodeon from 2002 to 2003.
And I have seen it.
There I was, innocently watching Sunday morning television while out of town this past weekend with friends and BOOM. Super Duper Sumos. I don't think we knew what we in for. Also according to wiki, "The main plot for the series was for the crimefighting sumo wrestlers to go on an adventure and fight using their buttocks." And you know what? That's pretty accurate.
There are three sumos. Boomam, the blonde white surfer dude, Kimo, the Japanese, Zen-like sumo with a strict honor code, and Mamoo, the black sensible, unofficial leader of the trio. And yes, I had to look up their names on the internet because while the show was airing the only thing I was thinking/saying was basically, "THIS CANNOT BE REAL."
But it was, you guys. It really was.
The plot of the episode we saw involved the guys going undercover at a ballet company. For some reason they disguised themselves as female ballet dancers and it was not until the very end that someone (the bad guy who might have been a flying monkey?) figured out they were actually sumos. It was...ridiculous. But also kind of awesome? I can't be sure.
Oh, and the BEST part is when the guys activate their super powers by bumping their asses together. A ray of light then shoots out (seriously) and they grow even bigger and more powerful (the sumos, not their asses). Chris, Andi, and I did out best to recreate this moment for you. No super powers (or rays of light) yet though.
I don't know when or even if this show airs anywhere else. I bet if you looked for it online you would find it, but be warned. It is craaaaaaaazy.