You didn't mention how much difference there is. That would determine what method to use. If it is slight, then scraping, sanding, etc. would work. However, if it is significant, by the time you get it flat you could loose too much of the overall thickness of the top. If it is significant, the only correct way would be to either break the glue joint or cut it along the glue line and re-glue.
Jim in Mayberry