Hi,
If you are able to I would recommend taking 6-9 credits in Graduate Courses ( as a non degree seeker), preferably getting A's. You can then include in the statement of intent why your grades suffered, and point out that you hope your graduate grades indicate your apititude for a graduate program.
I would also recommend doing human service work to display your experience in the field. A lot of schools want a well rounded student, not just someone with good grades.
I had a somewhat similar situation to you, and I was accepted into a couple of the top rated MSW programs in the country.
I hope this helps!!