I've been having a slight vibration tinking it was nothing but a tire balance problem.  I was wrong, it was the front driveshaft causing it.  I was under the Jeep the other day greasing everything when I noticed the slip joint had some play in it.  The splines were still tight if I tried to turn it, but when I pushed up on it the slop became apparent.

Driving home last night in the snow I decided to put it in fulltime 4WD, not good, the vibration was terrible!  I pulled the DS this morning and drove it, and there was no vibration, so I took it to a driveshaft shop.  They are going to cut the DS and replace the end, replace the splined yoke, and put a new u-joint in the pinion end.  Nothing was wrong with ANY of the u-joints, but he said he would replace it anyway.  The cardan joint was fine so I opted to save a few bucks and leave the u-joints in it alone.

Cost is $200 but I couldn't find any driveshafts locally. Besides it could just be as bad as the one I've got.  FYI, if you have vibrations, and can't seem to find what's causing them look at your slip joint.