In ACAD 2008 I can type in the relative path in the 'Found at' field which shown when you select the xref in 'EXTERNAL REFERENCES' dialog. When you hit enter the path in 'Saved Path' field will change to the relative path you entered and 'Found at' field will change to the full path of the xref if it is found in the relative path.
I found this way after I tried REFERENCE MANAGER which always freeze when I apply changes.
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