Does the vibration occur a a certain speed, only under load or at specific rpm range? Does it diminish when coasting only, or under engine braking? Noticed any oil leaks at the rear of the transfer case, or at the pinion seal on the rear diff? Just a thought, but checked the driveshaft yoke (at t-case ext. housing) for excess play? A worn bushing in the t-case will allow the slip yoke to wobble, and cause vibration/noise - although usually worse while coasting, not under load (in my experience). Both my front axle u-joints were gone @ 20K miles, but didn't cause vibration - just popping in turns. My driveshaft was out of balance from factory (had a slight bend too), which made a really irritating vibration at 65-68 mph. ... just my .02...<br>-Scott

 

You say 'it SEEMS to go away', and I'm sure you already checked. But check for mud or ice buildup on the inside of your rims. Sounds pretty trivial, but maybe worth a look before you spend too much time elsewhere.