To a large shallow bowl, add the cleaned shrimp, lime juice, season with salt to tasted, and place in the fridge for 30 to 60 minutes, or until shrimp are as 'cooked' as desired from the lime juice. While waiting, chop the other ingredients.
To a large bowl add the tomatoes, avocado, jalapeno, cucumber, red onion, cilantro, salt, pepper, and stir to combine.
Add the shrimp and lime juice to the bowl with the vegetables, stir to combine, and taste to check for seasoning balance. I prefer to add a bit more lime juice and am not shy about adding salt because it's what the ceviche needs to bring out the flavors in everything. Serve immediately.
Notes
Ceviche is best fresh, noting the shrimp will continue to 'cook' in the lime juice as time passes and so this is not a make ahead dish nor do I find the leftovers taste as good a day later.