Our team uses Subversion. It is supported by MPLAB 7.30 (just look in the help files under "Version Control" or something like that). The fact the machines are not regularly networked is not an issue. You create a repository on one machine with your base code, and all other machines can connect to it and perform what is called a "check out." You can then edit the code you checked out all you want, and later you reconnect to the repo and "commit" your changes. Performing an "update" allows you to receive the changes that others have made and integrate to the code on your machine. SVN (Subversion) has great internal differencing tools, and will be familiar to those who have worked with CVS. You can find it here:
http://subversion.tigris.org/
I also suggest using
http://tortoisesvn.tigris.org/ (like TortoiseCVS but for Subversion) and
http://winmerge.sourceforge.net/ (A powerful file-differ for Windows)
You can even convert existing CVS (which Subversion is designed to replace) repos to SVN using
http://cvs2svn.tigris.org/
Good Luck,
Robinson