get underneath car with vehicle weight on all 4 tires. either use a 4 post lift, a pit, or 4 ramps.
measure the angle of the tranny output shaft. this is easiest done by using a magnetic angle finder on the end of the output shaft. note the angle and orientation (uphill or downhill)
measure the pinion. this is easiest done by using a magnetic angle finder and putting a deep well socket in the yoke saddles, and putting the angle finder on the socket. note the angle and orientation (uphill or downhill)
ideally, under load you want the two shafts to be parallel. so for most leaf spring street cars, you want the pinion ~2 degrees nose down relative to the tranny output shaft. drag racers recommend 5-6, but in my experience, on a street car, that much pinion angle gives some nasty vibrations at highway speeds with part throttle cruise (you're maybe only using 30-40 HP, so you're not wrapping the springs as much)
so say the output shaft is 3 degrees downhill, like this: \.
to get "2 degrees nose down" you want the pinion to be 1 degree uphill, like this \, as 3 degrees uphill would be parallel to the tranny output shaft. say you measure yours, and it was 2 degrees downhill. you would then have 5 degrees negative pinion angle, and for a street car, you'd want to put some 3 degree shims in there to roll the pinion up to 1 degree uphill.
