Dissolve Jell-O in boiling water. Add pear juice, salt and vinegar. Pour 1/2-inch into a loaf pan. Chill until firm. Chill remaining Jell-O until cold and syrupy. Place in bowl of cracked ice and whip until fluffy like whipped cream. Cream cheese with ginger, fold in gelatin mixture gradually. Fold in pears. Pour over firm first layer. Chill until firm. Unmold on crisp lettuce. Garnish with mayonnaise.