Did you ever figure this out? I'm currently having the same issue with a 2006 TJ rubicon. Vibes at around 45MPH, put it in 4HI and the vibes go away put it back in 2 WD and and soon after there is a shudder and the vibes come back?
I currently have 2 new drive shafts, new transmission mounts and new axle bearings. Seems transfer case related?