The Driveline Angle idea is that the Driveshaft should face the face of the Differential {and I suppose the Center Bearing(s) and rear of Transmission} "Straight On." The concept is that the typical U-Joint speeds up and slows down as it rotates, UNLESS it's Straight On. That doesn't matter when the grocer's stock boy cranks the store awning up and down morning and night, but it does matter in a vehicle. That is why cars went to "CV" joints, the term being "Constant Velocity," to smooth thing out.
We had an OP who had this problem on a Ford-based C. I sent him some pix indicating that our differential was cocked upward to face the back end of the driveshaft straight on. I believe he went on to have a shop change the alignment of the rear axle.
I'm pretty sure that's Chevy meant by Driveline Angle, but there's another factor we should remember. A pieced shaft like we have is NOT supposed to be in a straight line front-to-rear as seen from above. It IS supposed to have a little zigzag in it. This would be seen by draping strings, weighted on both ends, over the shaft right at the "joints" {transmission, center bearing(s) and axle} then looking lengthwise along the shaft. The strings should NOT be in a perfect line. That said I don't know how much offset there should be.
