Place the first 4 ingredients in a medium bowl and whisk to combine. Mix in the shrimp, celery, bell pepper and green onion. Cut each avocado in half lengthwise; remove the pits. Trim a little from the bottom of each avocado so that they sit straight. Set a half avocado on each of 4 plates; brush with the lime juice. Divide and mound the shrimp mixture on top of the avocados and serve.