OK... it's best to measure caster when the vehicle is complete, sitting at ride height (ususally on its wheels), and on level ground.
Often times the stock springs set the caster are right where it needs to be. The perches on a Dana 30 are pretty thick, so if the caster is only off a degree or two, you can mark and carfully grind the perch mounting surface to add the angle required.
Vibration? Bump steer? Wandering? Read the article (sticky) on Steering, suspension, and driveline.