Can I select the latest draft of a given entry?
Something like this should work:

{% if craft.request.getParam('draft') == true %}
    {% set drafts = craft.entryRevisions.getDraftsByEntryId(entry.id) %}
    {% if drafts|length %}
        {% set entry = drafts[0] %}
    {% endif %}
{% endif %}

{{ entry.title }}
...


This is assuming of course that the entry is published at all. If it's disabled then the uri will return 404. In which case you would have to retrieve the entry in some other way. See this answer for an example.