Down to three SELECTS - much better. I couldn't get it to work as two.
SELECT KeyKey, max(F2.ValVal)
FROM (
SELECT KeyKey, F2.ValVal, max(F2.Prev) as Prev, F2.ValVal - max(F2.Prev) AS Diff
FROM(
SELECT tblData.KeyKey, tblData.ValVal AS ValVal, F1.ValVal as Prev
FROM tblData
LEFT JOIN (...