Yeah, I see a few problems here.
#1. You're probably upside down on your car, if you get rid of it, you're going to roll that negative equity into whatever you buy.
#2. SRT-8 Jeeps aren't cheap on the used market. Nevermind the fact that they get 14 mpg. I fail to see how you're going to save money by trading in the car your upside down on for something that's nowhere near economical.
#3. If you're going for practicality, you might as well go big or go home. There's no point in getting rid of the Mustang for something that's only marginally more practical. I'd go for a 4WD truck if that's your intent.
You don't care when my car was built or what options it has.