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Try something like the attached... by combining both lists (with the source of each list, Base 1 or Base 2, designated in a column) and then sorting by descending area code, you can use a Multi-Row tool to find the nearest match. If there is an exact match, it will use the exact match... if not, it will use the next highest number it finds (which it will carry down from the prior rows).
EDIT: Because @MarqueeCrew reads through things better than I do, and pointed out an error in my logic... :) If you want this to behave exactly like it does in Excel, where an approximate match is the previous one not the next one like my original example, then you can sort it by zip code ascending to accomplish the same thing for the closest match prior to the Base 1 zip code being looked up.
(Note: Example workflow attached is in version 11.7)