You might check the driveshaft u-joints, transfer case fluid, diff. fluid... Can you tell if the sound is coming from any location in particular, front, rear, center? Sometimes it can be difficult to identify where the noise is originating, but the more info you can offer, the better... How long has it been since the trans. fluid was changed, and the other fluids for that matter?
'72 CJ5 Ren
'91 XJ, '96 XJ, '98 XJ
'94 ZJ, '96 Orvis ZJ
'98 Platinum 5.9, '98 Slate 5.9 (DD)