The trouble with articles like the HotRod Hotline one, is it is written by yahoos that are not as careful as us yahoos try to be. To say truck needs to be on level ground is wrong and to start adding shims under transmission mount is wrong. (tilts the carb/float bowl out of level.) Maybe the motor shown is fuel injected. It also says one cause is "Improper phasing", it never mentions it again and does not say what it is. (It's the matching of the Ujoint yoke caps, VERY important!). You can install the slip yoke splines wrong and cause an out of phase condition. Many drive shafts and slip yokes are marked at the factory or shop to insure reinstall.
Albeit the blog is written to fix a vibration, it is not how to set up a motor/differential from scratch or adding/swapping one or the other as in our case here.
Folks are paid to write an article, they read a bunch of web bits and pieces, some of which is wrong and repeated over and over, and they put it all together in their own professional writing words...and it gets all screwed up.
Many articles talk about 3 deg, I don't know where they get that. GM has it's motors/tailshafts at 4 deg down from carb mount surface and we are supposed to parallel that at pinion. Forget 3 deg as a number for leaf spring trucks with stock Chevy motors. If it's a Chevy motor within our era's, and not at 4 deg, it's wrong. This is where level surface comes from. As Grigg said, the ujoint angles don't care about level ground. You are looking to match and be opposite. The level ground IS important when setting up a motor because the carb fuel level needs to be level to ground as we expect most driving done on level roads. It's a baseline for setting a Chevy motor. You set your chassis on level surface and check the intake manifold carb mounting surface for level or same bubble as garage floor. That's where that gets mixed up with pinion angle.
