I used to get that, but I cured it by letting the wife sit in the front seat seriously though, is it coming from underneath? if might be the gear/transfer box, mine did do that but I haven't noticed it recently, perhaps I'm just getting used to it.
Check your front wheel bearings, jack one wheel up and grab it 12-6, if it rocks then the chances are the bearing is shot, it could also be that you just need to repack the fronts with grease.
I get through front bearings on mine once a year, but that's due to the water getting in and me leaving it sat on the drive for 2 weeks without using it.
You should repack the front seals/hubs and do the seal kit, every 10-20,000 miles.
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