2000 Safari 70K miles floorboard vibration @ 70-80

I've got a 2000 Safari two wheel drive with around 71K miles on it. I've recently replaced the front brake rotors and pads, as well as put new tires on it (properly balanced on install).

A nagging vibration at highway speeds is bothersome (70-80 MPH). You don't really feel it in the steering wheel or the brake pedal, but you do feel it when you put your feet flat on the floor boards. You also feel it in your seat. It seems to have gotten worse in the last 10K miles or so, and I figured that the old tires were the source, but since I've put new tires on, it would seem to not be the case.

When I drive it on the express way, and put it in neutral, the vibration is unchanged.

Anyone else have something like this going on? Should I re-balance the drive shaft or something?

Well, I had my shop take a look at the truck to make sure that all the U-joints, drive shaft, etc. etc. was OK, and they all were. Kinda figured that.

But the darn thing still has that vibration, and it's really annoying when going down the highway with the cruise control set at that speed.

First thought was driveline, shifting to neutral usually takes driveline out of the picture if the vibration continues. If the vehicle is equipped with drum brakes on the rear, it's possible that a weight fell off of a drum. A close examination of the drums will determine if this is the case, (the weights are spot welded on at the factory). Excessive toe in or toe out can also cause a vibration that is usually felt in the steering wheel, but can also cause a vibration that is felt on the floor. Run your hands over the inside and outside edges of the shoulder of the tires, it should be a smooth wear pattern. If the pattern is jagged and symetrical, have the alignment checked. Hope this helps.