In a medium bowl, combine 1 container of plain yogurt with the envelope of ranch dressing mix. Remove half of the mixture to another bowl. Into one of the bowls, add the remaining container of plain yogurt and diced cucumber; mix well. Cover and refrigerate.
Preheat grill and lightly oil grate.
Mix the ground beef and 1/4 cup onion into the remaining half of the yogurt mixture.
Shape into 6 hamburger patties.
Grill patties on medium heat for 7 minutes each side, turning once.
Cut off 1/4 end of the pita pockets and fill with torn lettuce, grilled burger, creamy cucumber sauce and diced tomatoes.