Hey guys im kinda new with ios but i have been looking for a suitable solution to the freezing after unlocking in 5.1.1 with iPod Touch 4th Gen for a couple of months and i have found it and the only requirement is for your ios 5.1.1 to be Jailbroken , i only registered here to post the solution so you guys wont have to deal with this problem anymore !
All you have to do is install through Cydia the tweak Unfold 2.0.2 (add source if you dont have it , http://apt.thebigboss.org/repofiles/cydia
) , it changes the unlocking style by unfolding the wallpaper into parts(1 to 10) but it also eliminates the freezing problem !!! Now i can unlock everytime without worrying about freezing :-) it even showed me in a banner that i had unfolded it 500 times and if i would like to donate ! 500 times unlocked without freezing my Ipod touch 4gen.