This is hugely complicated, but if you are interested in the keeping the driveline as staight as it was at full throttle then here's how to do it, assuming you know the maximum output torque from the gearbox, and that the old setup was OK.
I'm assuming you've got a live rear axle
1) measure the rotational stiffness of the rear axle system, in terms of degrees per ft lb of wheel torque. call this k
2) measure the installed angle of the pinion nose in the original installation, with the wheels on the ground. call this alpha.
3) Call the old axle ratio R
4) call the new axle ratio N
5) call the maximum output torque from the gearbox T
Then the new installed angle = alpha-NTk/R
where - means anticlockwise viewed from the left
In more general terms it is hugely complicated, and as usual it's a compromise. In an 'ideal' system using the centreline of the output shaft of the gearbox as the datum, then you set the diff centre on that datum, then point the pinion nose down a bit to allow for windup under torque.
If you can't put the diff on the datum then it is a good idea to point the diff at the gearbox output shaft, and then drop the pinion nose a bit.
Rule of thumb: UJ angles less than 2 degrees for good refinement (actually need to be within 0.3 degrees for some apps). UJ angles not to exceed 4 degrees at maximum torque. UJ angle at no load could be -2 degrees (ie pinion points below gearbox), so giving you 6 degrees of windup, if you need that much. Half of all the above figures is a better bet.
LOL, to simplify what Greg said, try 5-7 degrees total down if you have leaf springs and a fairly serious engine, 3-5 down if it has leafs and a mild engine. For 4link or ladder bar systems run 1-2 degrees down. To measure, level the frame to the horizontal, and run your level on the face of the pinion yoke. While you are at it, make sure you verify your transmission inclination at the same time, as I am quite sure you dont have a stock powertrain, especially adding a 9".
Good luck!
Hey, that was the crudest approach that I thought might work straight off the drawing board. I can make it as complicated as you like (well actually I'd hand you over to someone who can work out the geometry of the UJs)! In production this stuff is pretty crucial - if you can keep UJs instead of CVs you've save 20 or 30 bucks a car, and got a more durable system. if you can keep a single piece driveshaft instead of a two piece that's about fifty buck a car, minimum, and a lot less trouble. That means we can throw 80 bucks into a car if we get these angles wrong- which would pay for a lot of bells and whistles.
My mechanic tells me that the recommended way to set the diff preload after dissassembly is to torque up the pinion nut until you get the same torque to turn as you originally had (our diff is set up using a collapsible spacer).
Demonstrably in our experience over the past year this is a bloody good way of introducing axle whine into a previously quiet diff, so the answer is no!
