It is only possible if you have 2 sets of saved SHSH Blobs for your device. The first set must be for any 4.x firmware. The second set must be for the specific version of IOS you want to downgrade to, which would normally be for 5.1.1.
The problem with SHSH Blobs is you cannot hack these. So to have them, you must have taken time to save them away when Apple were still providing them. In this scenario, that means you needed to save your 4.x blobs back in 2011, and your 5.1.1 blobs back in September 2012. There is simply no way round the SHSH Blob security, so if you didn't save them yourself back then, then you are totally out of luck.
If you think you may have them saved, you can follow the guide here to confirm exactly which ones you have -
http://www.ipadforums.net/jailbreak...01-how-save-your-shsh-blobs-tinyumbrella.html. But don't expect to get anything other than 6.0.1 if you have never done this before. To downgrade you must have both a 4.x set of blobs, and a 5.x set to downgrade with.
If you do happen to have both sets of blobs, you can follow my tutorial here to perform the downgrade (it is almost identical to the re-restore process in the guide) -
http://www.ipadforums.net/jailbreak...5-x-ipad2-ipad3-using-redsn0w-shsh-blobs.html
Hope that helps.