I found this PDF[0] that purports to be the first draft of the script, but I can't vouch for its authenticity. It was linked from this summary on a Pixar fan wiki[1].

[0] http://web.archive.org/web/20121213100913/http://www.raindan...

[1] http://pixar.wikia.com/wiki/Toy_Story_2_(original_storyline)