For a drag and drop interaction, the better bet is a priority. You would start with the priorities being 1, 2, 3, and so on, just like a sort order.
But then, the user wants to move item 5 between 1 and 2. Voila! Give it the value of 1.5. No other values need to change. The index update takes care of the rest.
For this to work, the priority needs to be stored as a floating point number. That might be an issue. Also, a sufficiently large number of changes could result in pushing the limits of floating point. So, if a user tries to take the last element and insert it between the first two, s/he can get away with it about few dozen times or so.
You can fix this with a process that periodically reassigns number for one (or all users, if in batch) starting at 1.
