If you keep the u-joint working angles around 3 degrees at both ends, you won't have any problem. The rear u-joint angle is always changing as you drive, way more then front, so being dead on is not all that important, get as close as practical and call it a day. The working angle of a u-joint is measured from output shaft to driveshaft, and driveshaft to input shaft. This eliminates any error in measurements when the truck is not level.
