If you have an FHA loan and you rent it out for 18 months or more and later you decide to try to short sale.... You will be denied! No ifs ands or buts!
You said you are underwater... You can short sale and buy another home immediately... with restrictions. One of the requirements is a distance factor another is you have to be current on your mortgage...
Search Short Sales and foreclosure Deals at www.BuyNjShortSales.com