...Grande Cherokee? My jeeps RPMs jump really high when it changes gears, and i took it to a mechanic and they told me that the transmission was fine. Could it be the oxygen sensor that controls the amount of air going to the engine?
I had a bad o2 sensor in my car too once. Make sure it isnt an electrical problem caused by a short. If your o2 sense is out, you might also be getting really really bad gas milage.