You either need to use Redsn0w 0.9.12b2 or Sn0wbreeze 2.9.6 to create a custom NO_BB 5.1.1 IPSW to restore with after putting the phone in pwnDFU or use Redsn0w 0.9.14b2 to downgrade the iPad BB to 05.13.04 to regain GPS, then you have the option of just installing stock 5.1.1 which will update the baseband to the latest which cannot be unlocked, or create a custom NO_BB 5.1.1 IPSW to update while preserving an unlockable baseband with GPS functionality. If you choose to not downgrade the baseband and update with a custom NO_BB ipsw, you will be stuck without GPS. If you choose to downgrade but do not use a custom NO_BB ipsw, you will be back to stock without any future errors like you've got and you will have the freedom to flash the iPad baseband again then downgrade the baseband to 05.13.04 to regain GPS and unlock at anytime you like. Just remember that anytime you have the iPad baseband, you must either downgrade the baseband to install a stock ipsw or use a custom NO_BB ipsw to update/restore. I would just downgrade the baseband to get GPS back, then update to 5.1.1 using a custom NO_BB ipsw to preserve the unlockable baseband.