Drive shaft - UJoint operating angle. The centerline of the transmission output shaft and the centerline of the pinion shaft in the rear end must be parallel with each other,
within 1/2 a degree or less, and that means parallel in BOTH planes: "up and down" and "side to side".
Videos that do a good job of explaining things:
Theory:
Theory:
HowTo:
Important. Nearly every source of reference/instructional material you find on the "inter-tubes" talks only about the vertical plane - the "up and down" angle - and this is the adjustment that "everyone" focuses on. However, even if you have this alignment perfect, if the alignment is off in the horizontal plane (side to side) you'll still get a vibration.
In other words, if things are "off" like this:
View attachment 154718
The two centerlines do not have to be pointed "directly at" each other, but they do have to be parallel within 1/2 a degree.
If they're off, you either have to move the tail of the transmission "sideways" to correct it, or you have to change the alignment of the rear axle.
Changing the alignment of the rear axle is the easier option of the two IF you have adjustable LOWER control arms. Always make these kinds of changes with the lowers, not the uppers.
The difficult part of alignment in the horizontal plane is getting an accurate measurement because your "up/down" angle finder can't help you with this one. You have to get creative and do something like using a plumb bob to create reference points on the floor (or perhaps on your lift ramps if you have a 4-post lift) at "both ends" of the car, or maybe "creative" use of one or more laser pointers, or using your high school geometry figure construction techniques --- something to construct two lines, one at the front, one at the rear, that are exactly 90-degrees to the crank/transmission centerline and the pinion centerline, then measuring the distance between the lines, front to back, on both sides.
Something like this:
View attachment 154719
You want the measurements on both sides to be the same.
As you can tell, if you don't get the two lines at the front and rear of the car "right", your long measurements on the sides won't tell you the truth.
If you end up having to make changes in either plane, it can affect the alignment in the other plane so it's best to keep measuring and adjusting BOTH until you get both of them right.
Yeah - it's a painstaking process, but worth it.
Bear