If it's already mixed with the background vocals, I wouldn't be surprised if the company did it in such a way that you can't completely remove them.
Generally, when mixing, the vocals (lead) are the only thing panned 50/50. Because of this, it's very easy to remove lead vocals for the purpose of karaoke. Listening to the backups, you might be able to do this. Nero used to package a tool with studio 4 or 5 for making karaoke CDs. They may still have the rights to that software.
Otherwise, I'll offer to make a background based on what you have and then never follow through. That's what friends are for.