well open up cydia and see if you have your shsh blobs saved for 5.1.1
if so the next step is to download redsnow b13 (i think you need that version), tiny umbrella, and then you also need to find a clean download of ios 5.1.1
at this point you are going to need to stitch together a custom firmware using redsnow, using the fresh clean 5.1.1 and your blobs which are saved on cydias servers, also in your case you need to preserve baseband. after the custom stitched firmware is created you are going to have to start itunes, youre going to have to start tiny umbrella and you are going to have to turn on the tss server.
at this point, again, using redsnow, put your phone into pwned dfu mode, then using itunes right click on restore and select your newly created custom stitched firmware. the restore will start, and PROBABLY will fail at the very end (THIS IS OK). after the restore completes use redsnow to kick your phone out of recovery, your phone will appear to reboot, and when it starts up again it will be on ios 5.1.1 with your preserved baseband. then jailbreak away.