DT, when i set up the truck, with all 4 wheels on ground, I measured at the trans shaft and diff yoke as you have, then installed driveshaft and measured its angle. trans at 4 degrees(down), diff at 4 degrees(up), driveshaft measured at 6 degrees, resulting in 2 degree u joint operating angle. rolls nice and smooth.

andy.

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84 D100 /6 long rod 225
86 D100 /6 my sons w/offy intake/weber carb
2016 buick regal turbo