> gem which tcltklib results in:
> C:/Ruby19/lib/ruby/site_ruby/1.9.1/i386-msvcrt/tcltklib.so

That's the built-in one, I think, not the gem one...and this is after a 
successful gem install?  "gem search tk_as_gem" lists something? what 
does "gem which tk" yield?

You might get some traction in building the gem on you 32 bit box and 
then copying over that entire ruby version to your 64 bit box, too.
GL!
-r
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