i'm going to play shogun for an hour and give you some links to pdfs that are helpful for diagnosing/troubleshooting (very) and mildly helpful for actual rebuilding.

here is the gm hydramatic/powertrain 100 page explanation of how the 4l30e functions


explains everything you could possibly want to know except how to take it apart 30mb
http://rs102.rapidshare.com/files/42743921/4L30E.PDF

transtech bulletin about rebuilding 4l30e (small)
http://www.transtec.com/rebuilder_news/2nd_q95r-n.pdf

and actually hosted by me (so that it won't likely disappear for a while)

the overdrive gasket instructions from bmw

http://plaza.ufl.edu/rroopnar/attachment%5b1%5d.pdf