Make sure that your Office suite is current with all of the updates, and then try it again, it should let you go over 2GB, although it is not really advisable to let your PST files get any larger than that, you run a higher risk of data corruption.
Hope this helps.
Also make sure that your hard drive is NTFS. I don’t think that a hard drive using the FAT file system will allow for files over 2 GB.
You can try PST Upgrade tool to convert ANSI PST to UNICODE successfully.