OK. I have a nasty vibration that begins at about 20 mph and peaks at 35 mph, resolving into a dull roar by the time I reach 55 mph. I've replaced tires and struts, didn't help. Oddly, the vibration will dissappear if I switch to 4wd and accelerate. When i stop accelerating the vibration returns. It is always present in 2wd. One of the univeral joints on the drive shaft (from the transfer case to the rear diff) is a bit loose, but not bad and I reckon the shaft isn't turning in 2wd anyway so this is (maybe?) not the problem. I understand there is a vacuum switch for the 4wd and an actuator but I can locate neither. Could this be teh source of teh vibration, as if somhow the car is running somewhere between 2wd and 4wd? Where are these vacuum bits? Help! I have to take the old girl to Maine (from Maryland) in June and would like to have some idea what is going on here. Thanks.