This is a very tough matter as there is no case law on this. Typically it is the approval of the landlord that you need. It sounds like the landlord will approve if all co-tenants approve. It also sounds as if you will not be able to pay your share of the rent much longer.
You need to sit down and meet with your roommates and make sure they understand that if they don't agree, they may have to pick up your share of the rent. I assume you plan to move out once the other person comes in.