I would suggest looking to see if your current mortgage is securitized by Fannie Mae or Freddie Mac at the following links (sorry can only post 1 link.....such a silly requirement):
Fannie Mae: http://www.fanniemae.com/loanlookup
Freddie Mac: google search Freddie Mac Loan Lookup, it will be the 1st link that is shown
If it is securitizied by one or the other, then you should be eligible for a HARP refinance (Fannie Mae - Refi Plus or DU Refi Plus and Freddie Mac - Open Access). When HARP 2.0 was released (March 2012 I believe) Fannie Mae and Freddie Mac imposed caps on the amount of adjustments you would face for credit score and level of financing. Thus, making it much more feasible for someone with a credit score less than 700 to take advantage of the program. And investment properties are eligible.
The one hang up that some lenders may have is your PMI. The PMI certificate has to be transferred to the new lender. Some lenders don't want to deal with it at all, and others will only deal with certain PMI companies. The big PMI companies (that lenders generally don't have an issue with) are: MGIC, Radian, United Guaranty.
I would suggest contacting a mortgage broker, who would have access to a number of different lenders and should be able to find someone to handle the refinance. Or you could contact your current lender and see if they have any options for you to refinance with the HARP program.
Hope that helps.