I have a 99 2WD with the 5.4, and at about 60,000 miles or so I heard a knocking noise coming out of the back end at slow speed (coasting between 10 and 20) that I figured out to be what I thought were loose wheel bearings. If you have the back end lifted off the ground you can grab the tire and shake it and it emits a hollow sounding noise. I had the wheel bearings looked at and they told me they were tight, and a used car salesman friend told that this noise is common among his trucks he has dealed with. He says that they get like that where they make noise and get sloppy, but they never seem to break, they just stay that way. I dont like hearing the noise cause it dosent sound very healthy, but I dont really know what else to do.