different ruby versions...different results

i just got a new mbp and started transferring some projects over.
when starting to use geokit on it, geocode_ip_address worked but not
on the imac.
differences are imac is running ruby 1.9.1p376 and mbp is running
ruby
1.8.7.
is there something in that difference that would cause geocode ip
address not to work on imac?
now that i see this working on the mbp, im concerned it will stop
working once i upgrade to 1.9.1.
it seems to be a to_yaml conversion for the cookie thats causing the
error.

id be curios to install 1.9.1 on the mbp if i could control which
version of ruby to use.
im kinda new at the whole ruby rails thing and unsure of how to load
and use different versions.

any suggestions?

so i went and followed hivelogic instructions on how to upgrade ruby:
http://hivelogic.com/articles/compiling-ruby-rubygems-and-rails-on-snow-leopard

and it went well...meaning which ruby returns /usr/local/bin/ruby
except ruby -v still returns 1.8.7 where sudo ruby -v returns 1.9.1

thats kinda what i wanted...maybe, im not sure
but how can i make the 1.9.1 the default version?
or...how can i manage my apps with two versions of ruby installed?

ruby -v # 1.8.7
sudo ruby -v # 1.9.1

whats going on here?