Transmission hesitation

6,000 miles and 6 months ago I replaced my transmission in my 1994 Grand Cherokee Laredo 4.0 4X4. The new unit was installed by a Jeep dealer and the unit is a factory rebuild. When I first start up when cold and shift into Drive or Reverse the vehicle does not move for about 15 to 30 seconds depending on how cold the outside temp. Other than this it operates beautifully. The fluid level is correct. LEL