Sure. Basically there are 2 additional columns in both tables that determine whether the row is valid or not. We'll call these columns X, and I was looking for a value of 0, so:
LEFT JOIN table1
ON table2.table2ID = table1.table2IDs
WHERE table2.X = 0
AND table1.X = 0;
What I was really looking for in this, were the rows in table2 that were not assigned in table1 (I guess I should note I pulled in the ID from table1 as well so I could sort for the nulls), but the join + checking for X=0 eliminated those rows, so taking "AND table1.X = 0" out of the query above made it work.
It's possible I got some invalid rows where the IDs matched, but since I was only looking for the nulls, that was fine.