May I have found the answer in what Jeep says in their catalog about:
Torque Converter Clutch
A featured designed to improve fuel economy is included in all automatic transmissions. A clutch within the torque converter engages automatically at a calibrated speed at light throttle. It engages at higher speeds under heavier acceleration. This may result in a slightly different feeling or response during normal operation in high gear. When the vehicle speed drops below a calibrated speed, or during acceleration, the clutch automatically and smoothly disengages. The feature is operational in Overdrive and in Drive.
NOTE: The torque converter clutch will not engage until the transmission fluid and engine coolant are warm [usually after 1-3 miles (1.6 4.8 km) if driving]. Because the engine speed is higher when the torque converter clutch is not engaged, it may seems as if the transmission is not shifting into Overdrive when cold. This is normal. Manually shifting between 3 (third gear) and 4 (fourth gear) positions (using the ERS shift control) will demonstrate that the transmission is able to shift into and out of Overdrive.
NOTE: If the vehicle has not been driven in several days, the first few seconds of operation after shifting the transmission into gear may seem sluggish. This is due to the fluid partially draining from the torque converter into the transmission. This condition is normal and will not cause damage to the transmission. The torque converter will refill within five seconds of shifting from Park into any other gear position.
Please inform me if it is the answer to my previous question about a strange behavior of the rear differential and if it is related to the rear differential sound.
Sunday, January 30th, 2011 AT 8:03 PM