How many miles on the car?
Could just be backlash in the drivetrain. Don't forget, with four wheel drive there are LOTs of extra parts underneath. Transfer case, second differential, two more drive axles. All these parts have some clearance built into them to allow them to work smoothly. With age comes wear so it might just be normal. New or unusual vibration would signal something to check.
Extra maintenance is is a cost of AWD.
(I know 'cause I've also got 2 Jeeps)