Upgrade jailbroken 3GS from 4.3.3 to 5.1.1

I've got a jailbroken 3GS on 4.3.3 that's been in a drawer for a while not being used. I want to keep the phone jailbroken and unlocked on an untethered jailbreak. From what i've read, 5.1.1 was the last untethered jailbreak for a 3GS with a new bootrom. How can I go about accomplishing this. I've created a custom IPSW to maintain my current base band so I can unlock the phone, but when I try to restore the phone to the new 5.1.1 ipsw I get an error from itunes...I'm guessing it has something to do with shsh's?? Can anyone provide any guidance on how I can achieve this? Thanx in advance. er