Well I took the front driveshaft to Pattersons Driveshafts here in Indy yesterday afternoon.  Turns out one of the u-joints in the cardan joint was going out, and the balance was off slightly.  I never could feel anything in it, but it showed up when he went to check the balance.

They rebuilt the cardan joint, and rebalanced the shaft.  I bolted it up, and drove it for awhile.  Not quite as smooth as 2WD, but nothing worth mentioning really.  I suppose 4WD will never be as smooth anyway.  But it DOES NOT vibrate the whole Jeep anymore.

You'd think a dealer would think to check stuff like that huh?  GOD how I hate dealers!

BTW, I had to drive the Jeep to Columbus Ohio last Friday to work there.  I got almost 18 mpg.  Would have been better but I was doing 70 mph until I got to Ohio.