There are three grease fittings on the drive shaft, not just the two at the U-Joints. Make sure all three are well filled. By doing this both my '99's stopped all clunking. Also, it wouldn't hurt to service all the driveline housings with state-of-the-art synthetic lubbricants, after a thorough flush of all and filter change at the tranny. This dramatically quieted-dowm my drive line.