I now have to parse out a response document and am not sure if Builder has some function to assist in this (which is the reverse of what I've been doing), or if I should use REXML, or even some other lib. REXML looks to do what I want, but I'm confused as to how it relates to Builder, and/or vice-versa, or if at all.
Builder and REXML aren't related.
That said, REXML is just the ticket for parsing XML. XmlSimple is also good, and I actually prefer it in some ways.
This Yahoo! article actually has a good overview of parsing XML in Ruby with examples using both REXML and XmlSimple, so you can compare the differences in flavour.