Thank you Gren_T. The vibration I am experiencing is coming from underneath (below the front seats) of the vehicle. When driving, there is no vibration on the steering wheel. When engine is working, the transfer case shift-handle vibrates slightly, and it's vibration become stronger when vehicle is moving. But I can feel it only by holding my hand on the tc shift-handle. The real vibration (from underneath, below the front seats) starts when the car reaches 50 miles per hour and it last up to 60 miles per hour. At the same time, the tc shift-handle vibrates according to RPM of the engine.