You'll see about 1 degree of angle change per additional inch of shackle length:
- http://www.4crawler.com/4x4/ForSale/Shackles.shtml#FAQ12

Shims are a more direct option:
- http://www.4crawler.com/4x4/ForSale/Shims.shtml

Or cut off the spring perches and relocate them to the correct angle.