Let's make sure you lubed the right part of the drive shaft. The U-joints need grease, but greasing the U-joints doesn't solve the problem, because that's not where the problem is.
See the attached .pdf file, which I scanned from the service manual.
The rear drive shaft is in two sections: a front section and a rear section. The rear section is in two parts, which fit together as a male/female joint with splines, just in front of the rear U-joint. I've circled that joint. As the springs flex and the axle moves up and down, the driveshaft grows longer and shorter, the length change being taken up by the splined joint.
The problem is that splined joint is dry. When you stop, the rear leaf springs wind up a bit, storing energy, and are held in that position by the brakes and the drive shaft. After a bit, that splined joint slips a bit, the springs unwind a bit, and the truck feels a bit as if it were bumped slightly from behind.
The cure is to grease that splined joint. The zerk for greasing it sticks out of the rear yoke like a sore thumb, right behind the splined joint. The cure is in the details about how you grease it. Give it about 25-35 squirts from the grease gun, watching the joint as you do. When you've greased it enough, it should start to push the two parts of the joint apart, and you can see it do so. This will begin to force the grease into the splines, and that is what solves the problem.
Is there a particular type of grease that should be used here? Curious as to what is specified by the service manual. Thanks!