My Ipad 3 have the same problem with you. I want to restore back to factory 5.1 but unfortunately, I didn't backup SHSH Blobs for to iOS 5.1. Now I will try to restore to 5.1.1 iOS
Could your please confirm that after restoring to iOS 5.1.1, no more jailbreak on your IPAD? ( by using the tutorial mentioned by Mickey330 )
Thanks in advance.