Combine the raw shrimp and lime juice in a large bowl, so lime juice covers shrimp completely. Let sit for 15 minutes, or until shrimp is no longer grey. Drain (DO NOT RINSE) shrimp and discard lime juice.
Combine the onion, cucumber, jalapeño, tomato and cilantro in a large bowl. Add the shrimp and mayo and stir to combine.
Serve on tostada shells with slices of avocado on top. Season with salt to taste. Enjoy!