Jack up the wheels, and spin them. find the one making the noise. Any vibration with this noise?
If you find a wheel that is making the noise, hold onto the spring with one hand while spinning the wheel with another. if the spring vibrates look strongly at the wheel bearings.
If none are making the noise, look at the drive shafts/u joints. See if the u joints have any play.
Now for the "I'm trying to help" disclaimer-
"I'm sure there are more possibilities as to the cause of this problem, this is not intended to be a "certain fix". its purely a suggestion, as to the first few things I'd look into if the problem were on my jeep."