Hi there,
I tried to attempt a similar thing with one of my reports a while ago, for the same reason. Unfortunately, to the best of my knowledge, it isn't possible to toggle the visibility of columns with the same toggle item as the drill down- at least after several days of searching MSDN there were no forthcoming solutions.
However, another way around the problem is to create a report parameter which toggles the drilldown and toggles the visibility of the columns you wish to hide, so when the user selects the parameter to drill down into the report the hidden columns are at the same time made visible.
I hope this is of some use to you.
Best regards,
Chris
Keep the rubber side down and the shiny side up.