We didn't really look at engine angle, it was what it was so to speak. But we did use the engine angle to set pinion. We used a digital angle finder on the output shaft of the trans and again on the pinion. It's easy to overthink driveline angles, you want the pinion and the trans angles to neutralize out with standard U joints but you need to consider what the suspension does under load to figure that out. You also need some angle so that the needle bearings in the u joints spin, if your diff aligns perfectly with your trans the bearings never get to do any work and sit stagnant (at least that's what the internet says and it makes sense to me). 4 link is likely going to be different than ladder bar/ leaf. I'm fairly familiar with jeep 4 link with a CV joint driveshaft and a leaf spring standard shaft. Trial by fire.
IIRC I think we're somewhere between 3-5 degrees down on the pinion likely closer to 5 which was the upper end of the allowable spectrum. With caltrac's you need some downward angle because as the rear end separates it rotates downward pivoting around the front spring eye reducing the pinion angle the more the diff travels. So at load/ speed when there is separation the pinion is rotated to a more correct neutral angle. We added a degree or two of tapered shim under the diff to take some angle out after modifying the rear suspension a bit with the sliders and new leaf springs. So far, no vibration cruising on the highway, under load, or in the shut down at the end of the track.
