Heather, there is another direction that you might want to investigate, unfortunately, it does involve removing the drywall on the common wall. Homasote makes a product called Soundasote specifically for this. They also make Homasote, which is a soft product that absorbs sound but not as much as Soundasote. You might check their web site (www.homasote.com) for some direction. I understand that Soundasote is hard to get in some areas but we did have good luck with putting Homasote in a basement stairway ceiling to keep washer/dryer noise from going all the way up to the second floor.
Another solution is to hire yourself a teenager who will play his music even louder than your neighbors do. Maybe that will force them into looking for another place.