You need to figure out how many payments you have left, or what the payout is right now, meaning what is the lump sum it would cost you to pay off your loan at this time.
It will be harder to get into something with a lower payment unless you have very little or negative equity.
Let's say you're $10k upside down. You'll have to add that $10k to a new car loan. Just the $10k with interest will cost you about $200/month, not even counting a new car loan that this would be tacked onto.
We need more info:
How many more payments you have left.
What interest rate you're paying on your loan.
How many miles on your Navigator.
It's condition (any accident history, etc)