1996 explorer 4x4 transfer case problem

My 1996 explorer will shift into 4 wheel drive but when you get up to 20 miles and hour it sounds like the transfer case is slipping and the the front wheels begin to stop the vehicle.

Does anyone know what the problem could be? The shift motor is operating fine.

Thank you

4 wheel drive is for snow and mud.on dry pavement you wil hear and feel vibration. does your suv have auto or 2 wheel?