Place the .urd file at the base of the work folder (i.e., C:\Library, D:\library, drive:\UR work\Library, etc.), then when importing folders/files that are in or under that path, UR will store the path relative. As long as those folder structures on the two computers match, things will match up properly.
Alternatively, you could
subst a drive/path on each computer to the same drive letter (i.e., subst x: c:\path\on\computera; subst x: d:\path\on\computerb [put this command in a .bat file and add a shortcut to that file in your Start Menu Startup folder to do this automatically]), and then link to files/paths on the subst'ed drive.