In a medium bowl or large (4-cup) measuring cup with a spout, whisk together strawberry jello mix and 1 cup of boiling water until jello is fully dissolved.
Once the jello is fully dissolved, stir in the cold water, lime juice, and rum. Mix until combined.
Use a strawberry corer or sharp paring knife to cut the stem from each strawberry and hollow out a bit of a cavity to fill with the jello. Shave off just a bit from the bottom of each strawberry so that it can stand up.
Carefully pour the jello mixture into each strawberry. You can use a food syringe or a squirt bottle, or simply pour with a steady hand.
Transfer jello shots to the refrigerator and chill for 2-3 hours until set. Just before serving, roll the top of each strawberry in granulated sugar and garnish with a very small lime wedge.