mtp, I was always under the impression that you had to have a bachelor's degree to join the peace corps, so I went to find the qualifications. You have probably seen this, but here is a "am I qualified" section. They say a 4 year degree isn't absolutely necessary for some programs, but can increase your chances of being accepted.
http://www.peacecorps.gov/index.cfm?...howvol.qualify
Good luck.