The quick answer.
For a single piece driveshaft the two yokes need to be orented the same way.
With the driveshaft out of the truck and the u-joints out you can slide a steel bar in each end, if one bar is level to the ground the other one better be also.

You can see if it is about right while still in the truck by just looking at it.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
---All pictures---
"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-