Dice garlic and add to peeled shrimp.
Melt margarine in large skillet.
Add shrimp and garlic, and sauté over medium heat until shrimp are just cooked, about 3 to 5 minutes.
Spread guacamole over half of each tortilla.
Top with 2 ounces each of Jack and Cheddar cheese.
Add one-sixth of the shrimp and garlic mixture and top each quesadilla with 2 tablespoons of salsa.
Fold other half of tortilla over ingredients to form a half-moon shape.
Grill on a barbecue or on a flat griddle until cheese is melted but not runny.
Flip and grill other side of tortilla until crisp.