The black stuff under the end of the tailpipe might just be condensation and soot, similar to wet stacking on a diesel engine. That was real common on older carburated cars in cold weather. In cold weather most car engines still run rich until they reach full operating temperature. Check under the oil fill cap for moisture. If there is a lot of condensation under that cap think about changing the oil or working the engine enough to evaporate the moisture out of the oil.
Your engine is starting to show some age, but with good maintanence it may have another 100,000 miles or more in it before it needs to be rebuilt. As others have said, see what it does in warm weather and until then enjoy your Jeep.