Loss of power over 45 mph
2003 SMB 4X4 w/ 5.4L engine 160,000 miles. The van has been running very well until two days ago. Yesterday there was a slight loss of power over 45 mph; under 45 mph it runs fine. At 50 mph and over, there is a new vibration or more of a "low growl" heard - something like a bad "U" joint. The "growl" is heard when the loss of power is felt. Cannot hear the vibration with the window rolled down. No vibration at idle, when stopped and no performance loss when accelerating. All fluids are good. Any ideas?