Do leg lifts. They are pretty minimal impact and give the abs a good work out. Your back will get a bit of a work out as well, so it may feel sore, but this is normal. You could also try asking a personal trainer at a gym for some tips (if you act like you are interested in a membership they will probably give you some advice for free).
Also, found this on Google, not sure if anything here will help ya out much, but it's worth a shot:
http://www.fitnesstipsforlife.com/ab...back-pain.html