I am having trouble driving my 2003 Nissan Murano over 65 mph on the highway. It revs really high and then engine sounds like it is going to explode. It seems like it is already in the highest gear possible and it of course won't shift any higher. I am afraid to take it out of town on longer highway trips because of this. I have an extended warranty but don't know what the problem is or if it would be covered. I have taken it to one Nissan Dealer and one Carmax dealer and neither have been able to diagnose a problem. They also aren't allowed to take it out on the highway which seems absurd. Any input would be much appreciated.