Stabilize your grill at a direct temperature of 500°F.
Slice and grill 2 lemons and 2 limes.
Place shrimp pieces in a non-reactive bowl, then juice all citrus (both grilled and non-grilled) over the shrimp. Make sure the shrimp are not packed too tightly in the juice and the liquid level is just above the level of the shrimp in the bowl. Transfer the bowl to a refrigerator and allow the citrus to cook the shrimp under refrigeration for 2 to 4 hours, depending on the size of your shrimp (the larger the shrimp pieces, the longer it will take to cook).
Once the shrimp is fully cooked, strain the liquid. Add the vegetables, sugar and salt to cooked shrimp pieces. Lastly, with a gentle hand, fold in the avocado and cilantro. Taste to see if you need to adjust the seasoning.
Place the Bibb lettuce leaves on a presentation plate and top with an appropriate amount of charred citrus ceviche to make your mock tacos. Enjoy!