Like Jay said, custom JavaScript verification should be able to handle this. In the ranking question's verification, you should see chunks of code labelled "Unanswer fake responses" and "Number ranked." Between these two chunks, please try adding this script:
// Require responses for item 1 - 5
for (var i = 1; i <= 5; i++) {
if (!SSI_GetValue('[% QuestionName() %]_' + i)) {
strErrorMessage = 'Error message...';
}
}