Does it seem to be coming from the front or rear?
If it's the front then get under the vehicle and grab the propshaft near one of the U/Js and try to move it up,down & side to side to feel for any play. Then do to other end of the propshaft. You may notice a small amount of movement on the output from the transfer box, this is normal.
Do the same on the rear but chock the wheels and release the handbrake.
If you don't find anything on the propshafts then check the wheel bearings by jacking one wheel, grab the tyre at the top & bottom and try to move it in & out. Any movement felt here will mean a loose wheel bearing.
