Traditionally, ceviche is made by letting the acid in a citrus juice cook the fish. For shrimp, however, you usually cook before adding to the usual ingredients.
The previous morning, we went to the FM and picked up some really good vine-ripe tomatoes, onions, garlic, cilantro, lemons for something else I made. So, with the leftover pico de gallo, I just adapted it and added the juice of about 4 or 5 lemons and the cooked shrimp. Just remember to never overcook your shrimp or else they get rubbery. I've posted my pico de gallo recipe before, so I'm not posting it again.
Bright, clean, very tasty.