If you need to move, could you afford the likely negative cash flow if you rented it out? If you rented it out, you might have a negative cash flow of $200, $300, or so a month. (Depends on your mortgage, your interest rate, what you could get in rent, etc.) If you can afford that, at least consider it.
If you need to sell, then you'd probably be looking at a short sale. You'd sell the property for what you could get--let's say $145,000. The sale would be contingent on the lender approving the sale. You'd have to show some "hardship"--job loss, illness, divorce, etc.--some reason why you wouldn't be able to continue making the payments. And a short sale would seriously injure your credit.
If you can't continue making payments, you'd be facing foreclosure.
Those are your basic options.