Got it to work... probably a kludge, but ...Summarized in T-SQL first:
SELECT SalesRep
    , SUM(Sales2016) AS Total2016
    , SUM(Sales2017) AS Total2017
    , SUM(Sales2017) - SUM(Sales2016) AS Delta
FROM
(SELECT SalesRep
    , SalesYear
--    , SalesYTD
    , CASE WHEN SalesYear = 2016 THEN SalesYTD ELSE 0 END As Sales2016
    , CASE WHEN SalesYear = 2017 THEN SalesYTD ELSE 0 END AS Sales2017
FROM dbo.TerritoryYTD) x
GROUP BY SalesRep;
Then in SSRS, I added a

tablix

(not a matrix). and added (SalesRep, SalesYear, Sales2016 and Sales2017).
Then I sorted the tablix on Sales2017 DESC.