Drivetrain shock while cruising 2001 Ranger Club Cab 4.0L, auto, part-time 4WD, 65K miles. Shock is like reving engine and dropping into gear. Very short duration (1 sec), amplitude varies, but always very noticeable, at worst very alarming. 4WD engaged or not; OD on (off?). No noticeable speed change. Occurs randomly in a given cruise at various speeds, so far 30-65mph. No lights flicker. Truck starts and runs perfectly otherwise. Big clue: Only happens when streets are wet. Has never happened on dry roads. Currently winter weather. Help!