2004 Dodge Durango Seemed Tranny Down Shifting Heard Clunk Check Engine Light

It seemed like the tranny was down shifting when I heard a clunk and then the check engine light came on. I didn't see any fluids under the Durango. I turned off the engine and it started right away, but the check engine light stayed on. None of the gauges went hot, or changed and it still runs good.
Help?

Thanks,
Dan