Mix sugar, melted butter, and crushed pretzels in a pie plate or medium baking dish. Press onto bottom and slightly up the sides.
Place the plate/dish into the fridge until ready to fill.
In a large bowl, mix sweetened condensed milk, crushed pineapple, crushed strawberries, and lime juice.
Stir the entire tub of Cool Whip into the bowl and mix well.
Pour the contents of the bowl into the pretzel crust. Depending on the size of your pie plate/dish, you may have enough mix to make 2 pies. I poured the extra mix I had into bowls for anyone who didn't want a pretzel crust, rather than making two pies.
Freeze the pie for at least 5 1/2 hours.
When ready to eat, thaw the pie out on the counter for 20-30 minutes before cutting.