How to Make It
Bring pineapple juice, sugar, and jalapeño to a boil in a small saucepan over medium-high. Reduce heat to medium-low, and simmer 3 minutes. Strain solids from juice; discard solids. Let cool completely in refrigerator, about 1 hour. Stir in lime juice.
Place tajin seasoning in a shallow dish. Run a lime wedge around the rim of 4 glasses; dip rim of each glass in tajin seasoning to coat. Fill each glass with ice. Divide tequila among each glass; top with some of the pineapple mixture. Pour margarita back and forth between 2 glasses to mix well. Pour margarita back in serving glass, and add more ice if necessary. Garnish with jalapeño slices and pineapple wedge, if desired.