You can look up the values here:
It looks like the mustang would be worth about twice what the jeep is,(Even though the Jeep is WAY better)
So if you want to go through all the trouble of trading and selling you could probably do Ok. The jeep will probably go for another 120k without any major problems, but you will have to start replacing things like the starter and alternator as they wear out, so you should factor that into your decision. Also, you may not like the design of the later Cherokees, so test one out first.