I was divorced January 2011. My exhusband and I owned a coop apartment. How do I get his name of the lease since I bought him out of the coop apt.?

My landlord refuses to take my husband''s name of the coop apt. lease until he signs a release form. The check clearly stated that the money was for the buyout of the apartment and my exhusband signed and cash the check. I have the copies. I''ve been trying to get him to sign the release form but he has refused. What can I do? The divorce does not stipulate that I am sole owner of the apartment.