In Rally you can rank Defects and User Stories but not Tasks (drag/drop doesn't work here).
Problem: DBA project has tasks that span nearly every Project. DBA manager can make a filter to see those tasks in the Task list view, but no apparent way to rank or prioritize them. There appears to be no Rank field for Tasks like there is for Defects, User Stories, and Test Cases.
Tasks can only be ranked in the context of the workitem to which they belong. It is to allow a user to order the tasks as they need to be done with the story, defect - on the collection page for tasks.