I have a bad vibration due to something in the rear driveshaft in my 1975 K20 longbed. The truck is smooth under 40 mph. At about 40 a vibration begins and gets really bad at 50 or so. I yanked the rear driveline and drove the truck in 4-HI and it is smooth as silk. No vibes at all. So I look at the rear driveshaft. This is a 1-piece shaft with no center support. The front U-Joint(near transfer case) is original and is 30 years old. The rear U-joint has been replaced at some time, but the grease seals on 3 of the 4 caps are ripped open and the roller bearings in the caps have started to wear into the inner surface they ride on. I also find that the slip yoke was installed improperly in relation to the front portion of the shaft. The slip yoke was 1 spline off from where it should have been, so the U-joints were phased slightly incorrectly. Trying to fix problems, I then installed 2 new U-Joints and changed the rotation of the rear portion of the slip yoke so the U-Joints were phased correctly. I did however notice that there is some side-play at the slip joint. Possibly because it was not greased regularly. So I re-installed the rear driveshaft and test drove it. The vibration is still there at 40 mph but it is less than it was. The vibration is only about 80% as strong as it was previously, so the new U-Joints and slip yoke adjustment must have helped some. What else can I do? I am thinking the looseness at the slip yoke may be contributing to a slight "Jumprope" effect at a higher driveshaft RPM. Is this possible? I was thinking of turning the driveshaft end for end and putting the slip yoke near the transfer case so the torsional loading on the splines of the slip yoke would be in the opposite direction of the current loading. Is this a bad idea? Does it matter if the splined slip yoke on the rear driveshaft is nearer to the rear axle or nearer to the Transfer case? Is one position better than the other? What was the factory position? Front or rear? Do you folks have any other suggestions on what to look for to cure this vibration? Thanks in advance, and sorry this was sooooo lengthy.