you can get 2*(and I think 4*) shims from any speedshop, 4 WD shop or mailorder catalog. If your pinion angle is out of alignment with your trans output shaft by more than 4* I would redo the spring perches.
Here is a link to the section in the KB with several articles that explain why proper phasing and angle is important.
http://www.hotrodders.com/kb/browsel...atid=55&page=2
Hope this helps,
mikey